diff --git a/.phpcq.yaml.dist b/.phpcq.yaml.dist
index 10be43eac..69a969bca 100644
--- a/.phpcq.yaml.dist
+++ b/.phpcq.yaml.dist
@@ -53,6 +53,7 @@ phpcq:
     # psalm
     - 8A03EA3B385DBAA1
     - 12CE0F1D262429A5
+    - 99BF4D9A33D65E1E
     # magl@magll.net
     - D2CCAC42F6295E7D
     # PHP_CodeSniffer
diff --git a/composer.json b/composer.json
index e8d5c798d..35a6fd281 100644
--- a/composer.json
+++ b/composer.json
@@ -92,7 +92,7 @@
   },
   "extra": {
     "branch-alias": {
-      "dev-release/2.4.0": "2.4.x-dev"
+      "dev-feature/symfony-7": "2.5.x-dev"
     },
     "contao-manager-plugin": "ContaoCommunityAlliance\\DcGeneral\\ContaoManager\\Plugin"
   }
diff --git a/src/Cache/Http/InvalidateCacheTags.php b/src/Cache/Http/InvalidateCacheTags.php
index d6b886127..3a9b51481 100644
--- a/src/Cache/Http/InvalidateCacheTags.php
+++ b/src/Cache/Http/InvalidateCacheTags.php
@@ -75,7 +75,7 @@ class InvalidateCacheTags implements InvalidateCacheTagsInterface
     public function __construct(
         string $namespace,
         EventDispatcherInterface $dispatcher,
-        CacheInvalidator $cacheManager = null
+        ?CacheInvalidator $cacheManager = null
     ) {
         $this->namespace    = $namespace;
         $this->dispatcher   = $dispatcher;
diff --git a/src/Contao/Compatibility/DcCompat.php b/src/Contao/Compatibility/DcCompat.php
index bafe5cd0e..7c5d0d8ce 100644
--- a/src/Contao/Compatibility/DcCompat.php
+++ b/src/Contao/Compatibility/DcCompat.php
@@ -61,7 +61,7 @@ class DcCompat extends General
      * @param ModelInterface|null  $model        The model within scope (optional).
      * @param string|null          $propertyName The name of the property within scope (optional).
      */
-    public function __construct(EnvironmentInterface $environment, ModelInterface $model = null, $propertyName = null)
+    public function __construct(EnvironmentInterface $environment, ?ModelInterface $model = null, ?string $propertyName = null)
     {
         // Prevent "Recoverable error: Argument X passed to SomClass::someMethod() must be an instance of DataContainer,
         // instance of ContaoCommunityAlliance\DcGeneral\Contao\Compatibility\DcCompat given" in callbacks.
diff --git a/src/Contao/Dca/Builder/Legacy/LegacyDcaDataDefinitionBuilder.php b/src/Contao/Dca/Builder/Legacy/LegacyDcaDataDefinitionBuilder.php
index cf55c9565..1784a7b5c 100644
--- a/src/Contao/Dca/Builder/Legacy/LegacyDcaDataDefinitionBuilder.php
+++ b/src/Contao/Dca/Builder/Legacy/LegacyDcaDataDefinitionBuilder.php
@@ -1203,6 +1203,10 @@ protected function parseModelOperations(Contao2BackendViewDefinitionInterface $v
         $collection = $view->getModelCommands();
 
         foreach ($operationsDca as $operationName => $operationDca) {
+            if (is_string($operationDca)) {
+                continue;
+            }
+            assert(is_array($operationDca));
             $command = $this->createCommand($operationName, $operationDca);
             $collection->addCommand($command);
         }
diff --git a/src/Contao/Picker/AbstractAwarePickerProvider.php b/src/Contao/Picker/AbstractAwarePickerProvider.php
index 83e25857c..0350de216 100644
--- a/src/Contao/Picker/AbstractAwarePickerProvider.php
+++ b/src/Contao/Picker/AbstractAwarePickerProvider.php
@@ -156,7 +156,7 @@ protected function getUser()
      *
      * @return array
      */
-    abstract protected function getRouteParameters(PickerConfig $config = null);
+    abstract protected function getRouteParameters(?PickerConfig $config = null);
 
     /**
      * Generates the URL for the picker.
diff --git a/src/Contao/Picker/PagePickerProvider.php b/src/Contao/Picker/PagePickerProvider.php
index 62c1175ae..25cfb7901 100644
--- a/src/Contao/Picker/PagePickerProvider.php
+++ b/src/Contao/Picker/PagePickerProvider.php
@@ -108,7 +108,7 @@ public function supportsValue(PickerConfig $config): bool
     /**
      * {@inheritdoc}
      */
-    public function getDcaTable(PickerConfig $config = null): string
+    public function getDcaTable(?PickerConfig $config = null): string
     {
         return 'tl_page';
     }
diff --git a/src/Contao/Picker/TreePickerProvider.php b/src/Contao/Picker/TreePickerProvider.php
index 41ee97365..87972bb0d 100644
--- a/src/Contao/Picker/TreePickerProvider.php
+++ b/src/Contao/Picker/TreePickerProvider.php
@@ -110,7 +110,7 @@ public function convertDcaValue(PickerConfig $config, mixed $value): int|string
     /**
      * {@inheritdoc}
      */
-    protected function getRouteParameters(PickerConfig $config = null)
+    protected function getRouteParameters(?PickerConfig $config = null)
     {
         return ['do' => 'tree'];
     }
diff --git a/src/Contao/Subscriber/FormatModelLabelSubscriber.php b/src/Contao/Subscriber/FormatModelLabelSubscriber.php
index 10fc1a424..2916f73fb 100644
--- a/src/Contao/Subscriber/FormatModelLabelSubscriber.php
+++ b/src/Contao/Subscriber/FormatModelLabelSubscriber.php
@@ -110,7 +110,7 @@ public function handleFormatModelLabel(FormatModelLabelEvent $event)
      *
      * @return string
      */
-    private function getFirstSorting(GroupAndSortingDefinitionInterface $sortingDefinition = null)
+    private function getFirstSorting(?GroupAndSortingDefinitionInterface $sortingDefinition = null)
     {
         if (null === $sortingDefinition) {
             return '';
diff --git a/src/Contao/View/Contao2BackendView/ActionHandler/MultipleHandler/OverrideAllHandler.php b/src/Contao/View/Contao2BackendView/ActionHandler/MultipleHandler/OverrideAllHandler.php
index 21b77017e..c655f457d 100644
--- a/src/Contao/View/Contao2BackendView/ActionHandler/MultipleHandler/OverrideAllHandler.php
+++ b/src/Contao/View/Contao2BackendView/ActionHandler/MultipleHandler/OverrideAllHandler.php
@@ -157,8 +157,8 @@ private function process(Action $action, EnvironmentInterface $environment)
      * @SuppressWarnings(PHPMD.LongVariable)
      */
     protected function handleInvalidPropertyValueBag(
-        PropertyValueBagInterface $propertyValueBag = null,
-        ModelInterface $model = null,
+        ?PropertyValueBagInterface $propertyValueBag,
+        ?ModelInterface $model,
         EnvironmentInterface $environment
     ) {
         // @codingStandardsIgnoreStart
@@ -208,17 +208,17 @@ protected function handleInvalidPropertyValueBag(
     /**
      * Handle override of model collection.
      *
-     * @param Action                    $action            The action.
-     * @param \ArrayObject              $renderInformation The render information.
-     * @param PropertyValueBagInterface $propertyValues    The property values.
-     * @param EnvironmentInterface      $environment       The environment.
+     * @param Action                         $action            The action.
+     * @param \ArrayObject                   $renderInformation The render information.
+     * @param PropertyValueBagInterface|null $propertyValues    The property values.
+     * @param EnvironmentInterface           $environment       The environment.
      *
      * @return void
      */
     private function handleOverrideCollection(
         Action $action,
         \ArrayObject $renderInformation,
-        PropertyValueBagInterface $propertyValues = null,
+        ?PropertyValueBagInterface $propertyValues,
         EnvironmentInterface $environment
     ) {
         if (!$propertyValues) {
diff --git a/src/Contao/View/Contao2BackendView/ActionHandler/ToggleHandler.php b/src/Contao/View/Contao2BackendView/ActionHandler/ToggleHandler.php
index 6d497bfd7..29dc1e0df 100644
--- a/src/Contao/View/Contao2BackendView/ActionHandler/ToggleHandler.php
+++ b/src/Contao/View/Contao2BackendView/ActionHandler/ToggleHandler.php
@@ -112,7 +112,7 @@ public function handleEvent(ActionEvent $event)
     protected function process(
         EnvironmentInterface $environment,
         ToggleCommandInterface $operation,
-        ModelIdInterface $modelId = null
+        ?ModelIdInterface $modelId = null
     ) {
         $dataProvider = $environment->getDataProvider();
         assert($dataProvider instanceof DataProviderInterface);
diff --git a/src/Contao/View/Contao2BackendView/ContaoWidgetManager.php b/src/Contao/View/Contao2BackendView/ContaoWidgetManager.php
index 22d3fd033..41c287e0c 100644
--- a/src/Contao/View/Contao2BackendView/ContaoWidgetManager.php
+++ b/src/Contao/View/Contao2BackendView/ContaoWidgetManager.php
@@ -272,8 +272,8 @@ protected function getUniqueId($propertyName)
     /**
      * Retrieve the instance of a widget for the given property.
      *
-     * @param string                    $property    Name of the property for which the widget shall be retrieved.
-     * @param PropertyValueBagInterface $inputValues The input values to use (optional) (RAW widget value format).
+     * @param string                         $property    Name of the property for which the widget shall be retrieved.
+     * @param PropertyValueBagInterface|null $inputValues The input values to use (optional) (RAW widget value format).
      *
      * @return Widget|null
      *
@@ -284,7 +284,7 @@ protected function getUniqueId($propertyName)
      * @SuppressWarnings(PHPMD.CamelCaseVariableName)
      * @SuppressWarnings(PHPMD.EmptyCatchBlock)
      */
-    public function getWidget($property, PropertyValueBagInterface $inputValues = null)
+    public function getWidget($property, ?PropertyValueBagInterface $inputValues = null)
     {
         $environment = $this->getEnvironment();
         $definition  = $environment->getDataDefinition();
@@ -401,16 +401,16 @@ protected function generateHelpText($property, Widget $widget)
     /**
      * Render the widget for the named property.
      *
-     * @param string                    $property     The name of the property for which the widget shall be rendered.
-     * @param bool                      $ignoreErrors Flag if the error property of the widget shall get
-     *                                                cleared prior rendering.
-     * @param PropertyValueBagInterface $inputValues  The input values to use (optional) (RAW widget value format).
+     * @param string                         $property     The name of the property for which the widget shall be rendered.
+     * @param bool                           $ignoreErrors Flag if the error property of the widget shall get
+     *                                                     cleared prior rendering.
+     * @param PropertyValueBagInterface|null $inputValues  The input values to use (optional) (RAW widget value format).
      *
      * @return string
      *
      * @throws DcGeneralRuntimeException For unknown properties.
      */
-    public function renderWidget($property, $ignoreErrors = false, PropertyValueBagInterface $inputValues = null)
+    public function renderWidget($property, $ignoreErrors = false, ?PropertyValueBagInterface $inputValues = null)
     {
         /** @var Widget $widget */
         $widget = $this->getWidget($property, $inputValues);
diff --git a/src/Contao/View/Contao2BackendView/Subscriber/WidgetBuilder.php b/src/Contao/View/Contao2BackendView/Subscriber/WidgetBuilder.php
index 1b7150540..b739e615e 100644
--- a/src/Contao/View/Contao2BackendView/Subscriber/WidgetBuilder.php
+++ b/src/Contao/View/Contao2BackendView/Subscriber/WidgetBuilder.php
@@ -109,7 +109,7 @@ class WidgetBuilder implements EnvironmentAwareInterface
     public function __construct(
         EnvironmentInterface $environment,
         TranslatorInterface $translator,
-        RequestScopeDeterminator $scopeDeterminator = null
+        ?RequestScopeDeterminator $scopeDeterminator = null
     ) {
         $this->environment = $environment;
         $this->translator  = $translator;
diff --git a/src/Controller/ControllerInterface.php b/src/Controller/ControllerInterface.php
index a37d036a2..997d49c77 100644
--- a/src/Controller/ControllerInterface.php
+++ b/src/Controller/ControllerInterface.php
@@ -182,11 +182,11 @@ public function getModelsFromClipboardItems(array $items);
      *
      * This will only return models, that are compatible with the current environment.
      *
-     * @param ModelIdInterface $parentModelId The optional parent id. If not given, the models must not have a parent.
+     * @param ModelIdInterface|null $parentModelId The optional parent id. If not given, the models must not have a parent.
      *
      * @return CollectionInterface
      */
-    public function getModelsFromClipboard(ModelIdInterface $parentModelId = null);
+    public function getModelsFromClipboard(?ModelIdInterface $parentModelId = null);
 
     /**
      * Evaluate clipboard items, then return the corresponding models.
@@ -201,11 +201,11 @@ public function getModelsFromClipboard(ModelIdInterface $parentModelId = null);
      * @return CollectionInterface
      */
     public function applyClipboardActions(
-        ModelIdInterface $source = null,
-        ModelIdInterface $after = null,
-        ModelIdInterface $into = null,
-        ModelIdInterface $parentModelId = null,
-        FilterInterface $filter = null,
+        ?ModelIdInterface $source = null,
+        ?ModelIdInterface $after = null,
+        ?ModelIdInterface $into = null,
+        ?ModelIdInterface $parentModelId = null,
+        ?FilterInterface $filter = null,
         array &$items = []
     );
 
@@ -218,7 +218,7 @@ public function applyClipboardActions(
      *
      * @return void
      */
-    public function pasteTop(CollectionInterface $models, $sortedBy, ModelIdInterface $parentId = null);
+    public function pasteTop(CollectionInterface $models, $sortedBy, ?ModelIdInterface $parentId = null);
 
     /**
      * Paste the content of the clipboard after the given model.
diff --git a/src/Controller/DefaultController.php b/src/Controller/DefaultController.php
index 138a1dd95..20bb8431a 100644
--- a/src/Controller/DefaultController.php
+++ b/src/Controller/DefaultController.php
@@ -275,7 +275,7 @@ public function assembleAllChildrenFrom($model, $providerName = '')
     protected function assembleSiblingsFor(
         ModelInterface $model,
         $sortingProperty = null,
-        ModelIdInterface $parentId = null
+        ?ModelIdInterface $parentId = null
     ) {
         // @codingStandardsIgnoreStart
         @trigger_error(
@@ -559,7 +559,7 @@ public function getModelsFromClipboardItems(array $items)
     /**
      * {@inheritDoc}
      */
-    public function getModelsFromClipboard(ModelIdInterface $parentModelId = null)
+    public function getModelsFromClipboard(?ModelIdInterface $parentModelId = null)
     {
         $environment = $this->getEnvironment();
 
@@ -588,12 +588,12 @@ public function getModelsFromClipboard(ModelIdInterface $parentModelId = null)
      * {@inheritDoc}
      */
     public function applyClipboardActions(
-        ModelIdInterface $source = null,
-        ModelIdInterface $after = null,
-        ModelIdInterface $into = null,
-        ModelIdInterface $parentModelId = null,
-        FilterInterface $filter = null,
-        array &$items = []
+        ?ModelIdInterface $source = null,
+        ?ModelIdInterface $after = null,
+        ?ModelIdInterface $into = null,
+        ?ModelIdInterface $parentModelId = null,
+        ?FilterInterface  $filter = null,
+        array &$items = [],
     ) {
         if ($source) {
             $actions = $this->getActionsFromSource($source, $parentModelId);
@@ -614,7 +614,7 @@ public function applyClipboardActions(
      *
      * @throws InvalidArgumentException When the model id is invalid.
      */
-    private function getActionsFromSource(ModelIdInterface $source, ModelIdInterface $parentModelId = null)
+    private function getActionsFromSource(ModelIdInterface $source, ?ModelIdInterface $parentModelId = null)
     {
         $definition = $this->getEnvironment()->getDataDefinition();
         assert($definition instanceof ContainerInterface);
@@ -662,7 +662,7 @@ private function getActionsFromSource(ModelIdInterface $source, ModelIdInterface
      *
      * @return array
      */
-    private function fetchModelsFromClipboard(FilterInterface $filter = null, ModelIdInterface $parentModelId = null)
+    private function fetchModelsFromClipboard(?FilterInterface $filter = null, ?ModelIdInterface $parentModelId = null)
     {
         $environment = $this->getEnvironment();
         assert($environment instanceof EnvironmentInterface);
@@ -721,9 +721,9 @@ private function fetchModelsFromClipboard(FilterInterface $filter = null, ModelI
      */
     private function doActions(
         array $actions,
-        ModelIdInterface $after = null,
-        ModelIdInterface $into = null,
-        ModelIdInterface $parentModelId = null,
+        ?ModelIdInterface $after = null,
+        ?ModelIdInterface $into = null,
+        ?ModelIdInterface $parentModelId = null,
         array &$items = []
     ) {
         if ($parentModelId) {
@@ -765,7 +765,7 @@ private function doActions(
      * @return void
      *
      */
-    private function applyAction(array &$action, array &$deepCopyList, ModelInterface $parentModel = null)
+    private function applyAction(array &$action, array &$deepCopyList, ?ModelInterface $parentModel = null)
     {
         /** @var ModelInterface|null $model */
         $model = $action['model'];
@@ -842,7 +842,7 @@ private function doCloneAction(ModelInterface $model)
      *
      * @return void
      */
-    private function ensureSameGrouping(array $actions, ModelIdInterface $after = null)
+    private function ensureSameGrouping(array $actions, ?ModelIdInterface $after = null)
     {
         $environment  = $this->getEnvironment();
         $groupingMode = ViewHelpers::getGroupingMode($environment);
@@ -876,9 +876,9 @@ private function ensureSameGrouping(array $actions, ModelIdInterface $after = nu
      */
     private function sortAndPersistModels(
         array $actions,
-        ModelIdInterface $after = null,
-        ModelIdInterface $into = null,
-        ModelIdInterface $parentModelId = null,
+        ?ModelIdInterface $after = null,
+        ?ModelIdInterface $into = null,
+        ?ModelIdInterface $parentModelId = null,
         array &$items = []
     ) {
         $models = $this->createModelCollectionFromActions($actions, $items);
@@ -906,7 +906,7 @@ private function sortAndPersistModels(
      *
      * @return void
      */
-    private function processPasteAfter(CollectionInterface $models, ModelIdInterface $after = null)
+    private function processPasteAfter(CollectionInterface $models, ?ModelIdInterface $after = null)
     {
         if ($after && $models->count() && $after->getId()) {
             $manualSorting = ViewHelpers::getManualSortingProperty($this->getEnvironment());
@@ -930,7 +930,7 @@ private function processPasteAfter(CollectionInterface $models, ModelIdInterface
      *
      * @return void
      */
-    private function processPasteInto(CollectionInterface $models, ModelIdInterface $into = null)
+    private function processPasteInto(CollectionInterface $models, ?ModelIdInterface $into = null)
     {
         if ($into && $models->count() && $into->getId()) {
             $manualSorting = ViewHelpers::getManualSortingProperty($this->getEnvironment());
@@ -958,9 +958,9 @@ private function processPasteInto(CollectionInterface $models, ModelIdInterface
      */
     private function processPasteTopWithoutReference(
         CollectionInterface $models,
-        ModelIdInterface $after = null,
-        ModelIdInterface $into = null,
-        ModelIdInterface $parent = null
+        ?ModelIdInterface $after = null,
+        ?ModelIdInterface $into = null,
+        ?ModelIdInterface $parent = null
     ) {
         if (
             $models->count()
@@ -987,12 +987,12 @@ private function processPasteTopWithoutReference(
     /**
      * Process paste the content of the clipboard onto the top after a model.
      *
-     * @param CollectionInterface $models The collection of models.
-     * @param ModelIdInterface    $parent The parent model id.
+     * @param CollectionInterface   $models The collection of models.
+     * @param ModelIdInterface|null $parent The parent model id.
      *
      * @return void
      */
-    private function processPasteTopAfterModel(CollectionInterface $models, ModelIdInterface $parent = null)
+    private function processPasteTopAfterModel(CollectionInterface $models, ?ModelIdInterface $parent = null)
     {
         if ($parent && $models->count()) {
             $manualSorting = ViewHelpers::getManualSortingProperty($this->getEnvironment());
@@ -1177,7 +1177,7 @@ protected function doDeepCopy(array $deepCopyList)
     /**
      * {@inheritDoc}
      */
-    public function pasteTop(CollectionInterface $models, $sortedBy, ModelIdInterface $parentId = null)
+    public function pasteTop(CollectionInterface $models, $sortedBy, ?ModelIdInterface $parentId = null)
     {
         $environment = $this->getEnvironment();
 
diff --git a/src/Controller/ModelCollector.php b/src/Controller/ModelCollector.php
index 58719ffd3..6a5882b4a 100644
--- a/src/Controller/ModelCollector.php
+++ b/src/Controller/ModelCollector.php
@@ -352,7 +352,7 @@ public function searchParentFromHierarchical(ModelInterface $model): ?ModelInter
     public function collectSiblingsOf(
         ModelInterface $model,
         $sortingProperty = null,
-        ModelIdInterface $parentId = null
+        ?ModelIdInterface $parentId = null
     ) {
         $registry = $this->environment->getBaseConfigRegistry();
         assert($registry instanceof BaseConfigRegistryInterface);
diff --git a/src/Controller/SortingManager.php b/src/Controller/SortingManager.php
index 120cdd39a..363cd5505 100644
--- a/src/Controller/SortingManager.php
+++ b/src/Controller/SortingManager.php
@@ -96,10 +96,10 @@ class SortingManager
      *                                                the collection.
      */
     public function __construct(
-        CollectionInterface $models = null,
-        CollectionInterface $siblings = null,
-        string $sortedBy = null,
-        ModelInterface $previousModel = null
+        ?CollectionInterface $models = null,
+        ?CollectionInterface $siblings = null,
+        ?string $sortedBy = null,
+        ?ModelInterface $previousModel = null
     ) {
         if ($models) {
             $this->setModels($models);
diff --git a/src/DC/General.php b/src/DC/General.php
index 6d3c86729..12963a6da 100644
--- a/src/DC/General.php
+++ b/src/DC/General.php
@@ -76,7 +76,7 @@ class General extends DataContainer implements DataContainerInterface
      * @SuppressWarnings(PHPMD.Superglobals)
      * @SuppressWarnings(PHPMD.UnusedFormalParameter)
      */
-    public function __construct($tableName, array $module = [], CacheInterface $cache = null)
+    public function __construct($tableName, array $module = [], ?CacheInterface $cache = null)
     {
         // Prevent "Recoverable error: Argument X passed to SomClass::someMethod() must be an instance of DataContainer,
         // instance of ContaoCommunityAlliance\DcGeneral\DC_General given" in callbacks.
diff --git a/src/DataDefinition/Palette/Condition/Palette/DefaultPaletteCondition.php b/src/DataDefinition/Palette/Condition/Palette/DefaultPaletteCondition.php
index ffee0956f..d04e0aedb 100644
--- a/src/DataDefinition/Palette/Condition/Palette/DefaultPaletteCondition.php
+++ b/src/DataDefinition/Palette/Condition/Palette/DefaultPaletteCondition.php
@@ -32,7 +32,7 @@ class DefaultPaletteCondition implements PaletteConditionInterface
     /**
      * {@inheritdoc}
      */
-    public function getMatchCount(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getMatchCount(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         return 0;
     }
diff --git a/src/DataDefinition/Palette/Condition/Palette/PaletteConditionChain.php b/src/DataDefinition/Palette/Condition/Palette/PaletteConditionChain.php
index 51e4866af..866783796 100644
--- a/src/DataDefinition/Palette/Condition/Palette/PaletteConditionChain.php
+++ b/src/DataDefinition/Palette/Condition/Palette/PaletteConditionChain.php
@@ -38,7 +38,7 @@ class PaletteConditionChain extends AbstractConditionChain implements PaletteCon
      * @throws DcGeneralRuntimeException When a condition that does not implement PaletteConditionInterface
      *                                   is encountered.
      */
-    public function getMatchCount(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getMatchCount(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         $totalCount = false;
 
diff --git a/src/DataDefinition/Palette/Condition/Palette/PaletteConditionInterface.php b/src/DataDefinition/Palette/Condition/Palette/PaletteConditionInterface.php
index 1e57e5e37..ecb929ba9 100644
--- a/src/DataDefinition/Palette/Condition/Palette/PaletteConditionInterface.php
+++ b/src/DataDefinition/Palette/Condition/Palette/PaletteConditionInterface.php
@@ -46,7 +46,7 @@ interface PaletteConditionInterface extends ConditionInterface
      *
      * @return false|int
      */
-    public function getMatchCount(ModelInterface $model = null, PropertyValueBag $input = null);
+    public function getMatchCount(?ModelInterface $model = null, ?PropertyValueBag $input = null);
 
     /**
      * Create a deep clone of the condition.
diff --git a/src/DataDefinition/Palette/Condition/Palette/PropertyFalseCondition.php b/src/DataDefinition/Palette/Condition/Palette/PropertyFalseCondition.php
index eac3cb871..33e81b564 100644
--- a/src/DataDefinition/Palette/Condition/Palette/PropertyFalseCondition.php
+++ b/src/DataDefinition/Palette/Condition/Palette/PropertyFalseCondition.php
@@ -32,7 +32,7 @@ class PropertyFalseCondition extends AbstractBoolPaletteCondition
     /**
      * {@inheritdoc}
      */
-    public function getMatchCount(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getMatchCount(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         if (!$this->propertyName) {
             return false;
diff --git a/src/DataDefinition/Palette/Condition/Palette/PropertyTrueCondition.php b/src/DataDefinition/Palette/Condition/Palette/PropertyTrueCondition.php
index fcb271e06..556389f8a 100644
--- a/src/DataDefinition/Palette/Condition/Palette/PropertyTrueCondition.php
+++ b/src/DataDefinition/Palette/Condition/Palette/PropertyTrueCondition.php
@@ -32,7 +32,7 @@ class PropertyTrueCondition extends AbstractBoolPaletteCondition
     /**
      * {@inheritdoc}
      */
-    public function getMatchCount(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getMatchCount(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         if (!$this->propertyName) {
             return false;
diff --git a/src/DataDefinition/Palette/Condition/Palette/PropertyValueCondition.php b/src/DataDefinition/Palette/Condition/Palette/PropertyValueCondition.php
index c6c2843aa..c7fe4ad7e 100644
--- a/src/DataDefinition/Palette/Condition/Palette/PropertyValueCondition.php
+++ b/src/DataDefinition/Palette/Condition/Palette/PropertyValueCondition.php
@@ -141,7 +141,7 @@ public function getStrict()
     /**
      * {@inheritdoc}
      */
-    public function getMatchCount(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getMatchCount(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         if (!$this->propertyName) {
             return false;
diff --git a/src/DataDefinition/Palette/Condition/Property/BooleanCondition.php b/src/DataDefinition/Palette/Condition/Property/BooleanCondition.php
index 889dd176d..b3064f3fd 100644
--- a/src/DataDefinition/Palette/Condition/Property/BooleanCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/BooleanCondition.php
@@ -79,10 +79,10 @@ public function getValue()
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         return $this->value;
     }
diff --git a/src/DataDefinition/Palette/Condition/Property/DumpingPropertyCondition.php b/src/DataDefinition/Palette/Condition/Property/DumpingPropertyCondition.php
index 8a04ce575..dba8fbe2e 100644
--- a/src/DataDefinition/Palette/Condition/Property/DumpingPropertyCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/DumpingPropertyCondition.php
@@ -59,10 +59,10 @@ public function __construct($propertyCondition)
      * @psalm-suppress ForbiddenCode - We explicitly allow var_dump() here for debugging purposes.
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         $result = $this->propertyCondition->match($model, $input, $property, $legend);
 
diff --git a/src/DataDefinition/Palette/Condition/Property/NotCondition.php b/src/DataDefinition/Palette/Condition/Property/NotCondition.php
index 32d61dbc4..39dbdea6b 100644
--- a/src/DataDefinition/Palette/Condition/Property/NotCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/NotCondition.php
@@ -75,10 +75,10 @@ public function getCondition()
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         return !$this->condition->match($model, $input, $property, $legend);
     }
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyCallbackCondition.php b/src/DataDefinition/Palette/Condition/Property/PropertyCallbackCondition.php
index d7637a514..459988c67 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyCallbackCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyCallbackCondition.php
@@ -52,10 +52,10 @@ public function __construct($callback)
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         return \call_user_func($this->callback, $model, $input, $property, $legend);
     }
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyConditionChain.php b/src/DataDefinition/Palette/Condition/Property/PropertyConditionChain.php
index 4d662ce04..bfe91cb0b 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyConditionChain.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyConditionChain.php
@@ -40,10 +40,10 @@ class PropertyConditionChain extends AbstractConditionChain implements PropertyC
      *                                   is encountered.
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         if (static::AND_CONJUNCTION === $this->conjunction) {
             foreach ($this->conditions as $condition) {
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyConditionInterface.php b/src/DataDefinition/Palette/Condition/Property/PropertyConditionInterface.php
index e9ff3aadc..4a400742e 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyConditionInterface.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyConditionInterface.php
@@ -48,10 +48,10 @@ interface PropertyConditionInterface extends ConditionInterface
      * @return bool
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     );
 
     /**
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyEditableCondition.php b/src/DataDefinition/Palette/Condition/Property/PropertyEditableCondition.php
index ecda816c5..10820cb6f 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyEditableCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyEditableCondition.php
@@ -76,10 +76,10 @@ public function getPropertyName()
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         if (!$legend) {
             return false;
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyFalseCondition.php b/src/DataDefinition/Palette/Condition/Property/PropertyFalseCondition.php
index 9bee678b0..2c92d4926 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyFalseCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyFalseCondition.php
@@ -38,10 +38,10 @@ class PropertyFalseCondition implements PropertyConditionInterface
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         if ($input && $input->hasPropertyValue($this->propertyName)) {
             $value = $input->getPropertyValue($this->propertyName);
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyTrueCondition.php b/src/DataDefinition/Palette/Condition/Property/PropertyTrueCondition.php
index 336166504..abac246b9 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyTrueCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyTrueCondition.php
@@ -38,10 +38,10 @@ class PropertyTrueCondition implements PropertyConditionInterface
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         if ($input && $input->hasPropertyValue($this->propertyName)) {
             $value = $input->getPropertyValue($this->propertyName);
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyValueCondition.php b/src/DataDefinition/Palette/Condition/Property/PropertyValueCondition.php
index a37c45dca..edab343db 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyValueCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyValueCondition.php
@@ -145,10 +145,10 @@ public function getStrict()
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         if ($input && $input->hasPropertyValue($this->propertyName)) {
             $value = $input->getPropertyValue($this->propertyName);
diff --git a/src/DataDefinition/Palette/Condition/Property/PropertyVisibleCondition.php b/src/DataDefinition/Palette/Condition/Property/PropertyVisibleCondition.php
index 0f0ba0358..395dbae23 100644
--- a/src/DataDefinition/Palette/Condition/Property/PropertyVisibleCondition.php
+++ b/src/DataDefinition/Palette/Condition/Property/PropertyVisibleCondition.php
@@ -75,10 +75,10 @@ public function getPropertyName()
      * {@inheritdoc}
      */
     public function match(
-        ModelInterface $model = null,
-        PropertyValueBag $input = null,
-        PropertyInterface $property = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBag $input = null,
+        ?PropertyInterface $property = null,
+        ?LegendInterface $legend = null
     ) {
         if (!$legend) {
             return false;
diff --git a/src/DataDefinition/Palette/Legend.php b/src/DataDefinition/Palette/Legend.php
index 3f4378e06..7ea2ade02 100644
--- a/src/DataDefinition/Palette/Legend.php
+++ b/src/DataDefinition/Palette/Legend.php
@@ -74,7 +74,7 @@ public function __construct($name)
     /**
      * {@inheritdoc}
      */
-    public function setPalette(PaletteInterface $palette = null)
+    public function setPalette(?PaletteInterface $palette = null)
     {
         if ($this->palette) {
             $this->palette->removeLegend($this);
@@ -148,7 +148,7 @@ public function setProperties(array $properties)
     /**
      * {@inheritdoc}
      */
-    public function addProperties(array $properties, PropertyInterface $before = null)
+    public function addProperties(array $properties, ?PropertyInterface $before = null)
     {
         foreach ($properties as $property) {
             $this->addProperty($property, $before);
@@ -161,7 +161,7 @@ public function addProperties(array $properties, PropertyInterface $before = nul
      *
      * @throws DcGeneralInvalidArgumentException When the property passed as $before can not be found.
      */
-    public function addProperty(PropertyInterface $property, PropertyInterface $before = null)
+    public function addProperty(PropertyInterface $property, ?PropertyInterface $before = null)
     {
         $hash = \spl_object_hash($property);
 
diff --git a/src/DataDefinition/Palette/LegendInterface.php b/src/DataDefinition/Palette/LegendInterface.php
index 2a7a38cc2..d84f25b94 100644
--- a/src/DataDefinition/Palette/LegendInterface.php
+++ b/src/DataDefinition/Palette/LegendInterface.php
@@ -38,7 +38,7 @@ interface LegendInterface
      *
      * @return LegendInterface
      */
-    public function setPalette(PaletteInterface $palette = null);
+    public function setPalette(?PaletteInterface $palette = null);
 
     /**
      * Return the palette this legend belongs to.
@@ -99,22 +99,22 @@ public function setProperties(array $properties);
      * Add all properties to this legend.
      *
      * @param array|PropertyInterface[] $properties The properties.
-     * @param PropertyInterface         $before     The property before the passed properties shall be inserted
+     * @param PropertyInterface|null    $before     The property before the passed properties shall be inserted
      *                                              (optional).
      *
      * @return LegendInterface
      */
-    public function addProperties(array $properties, PropertyInterface $before = null);
+    public function addProperties(array $properties, ?PropertyInterface $before = null);
 
     /**
      * Add a property to this legend.
      *
-     * @param PropertyInterface $property The property.
-     * @param PropertyInterface $before   The property before the passed property shall be inserted (optional).
+     * @param PropertyInterface      $property The property.
+     * @param PropertyInterface|null $before   The property before the passed property shall be inserted (optional).
      *
      * @return LegendInterface
      */
-    public function addProperty(PropertyInterface $property, PropertyInterface $before = null);
+    public function addProperty(PropertyInterface $property, ?PropertyInterface $before = null);
 
     /**
      * Remove a property from this legend.
diff --git a/src/DataDefinition/Palette/Palette.php b/src/DataDefinition/Palette/Palette.php
index 67edab7ee..35b7c65d3 100644
--- a/src/DataDefinition/Palette/Palette.php
+++ b/src/DataDefinition/Palette/Palette.php
@@ -75,7 +75,7 @@ public function getName()
     /**
      * {@inheritdoc}
      */
-    public function getProperties(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getProperties(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         $properties = [[]];
         foreach ($this->legends as $legend) {
@@ -88,7 +88,7 @@ public function getProperties(ModelInterface $model = null, PropertyValueBag $in
     /**
      * {@inheritdoc}
      */
-    public function getVisibleProperties(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getVisibleProperties(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         $properties = [];
         foreach ($this->getLegends() as $legend) {
@@ -105,7 +105,7 @@ public function getVisibleProperties(ModelInterface $model = null, PropertyValue
     /**
      * {@inheritdoc}
      */
-    public function getEditableProperties(ModelInterface $model = null, PropertyValueBag $input = null)
+    public function getEditableProperties(?ModelInterface $model = null, ?PropertyValueBag $input = null)
     {
         $properties = [];
         foreach ($this->getLegends() as $legend) {
@@ -164,7 +164,7 @@ public function setLegends(array $legends)
     /**
      * {@inheritdoc}
      */
-    public function addLegends(array $legends, LegendInterface $before = null)
+    public function addLegends(array $legends, ?LegendInterface $before = null)
     {
         foreach ($legends as $legend) {
             $this->addLegend($legend, $before);
@@ -200,7 +200,7 @@ public function containsLegend(LegendInterface $legend)
      *
      * @throws DcGeneralInvalidArgumentException When the legend passed as $before can not be found.
      */
-    public function addLegend(LegendInterface $legend, LegendInterface $before = null)
+    public function addLegend(LegendInterface $legend, ?LegendInterface $before = null)
     {
         $hash = \spl_object_hash($legend);
 
@@ -273,7 +273,7 @@ public function getLegends()
     /**
      * {@inheritdoc}
      */
-    public function setCondition(PaletteConditionInterface $condition = null)
+    public function setCondition(?PaletteConditionInterface $condition = null)
     {
         $this->condition = $condition;
 
diff --git a/src/DataDefinition/Palette/PaletteCollection.php b/src/DataDefinition/Palette/PaletteCollection.php
index 7227a9e88..777705d1b 100644
--- a/src/DataDefinition/Palette/PaletteCollection.php
+++ b/src/DataDefinition/Palette/PaletteCollection.php
@@ -110,7 +110,7 @@ public function hasPalette(PaletteInterface $palette)
      * @throws DcGeneralInvalidArgumentException Is thrown if there is no palettes found.
      * @throws DcGeneralInvalidArgumentException Is thrown if there is no palette found or more than one palette.
      */
-    public function findPalette(ModelInterface $model = null, PropertyValueBagInterface $input = null)
+    public function findPalette(?ModelInterface $model = null, ?PropertyValueBagInterface $input = null)
     {
         $matches = [];
 
diff --git a/src/DataDefinition/Palette/PaletteCollectionInterface.php b/src/DataDefinition/Palette/PaletteCollectionInterface.php
index a499b0a19..c7af88729 100644
--- a/src/DataDefinition/Palette/PaletteCollectionInterface.php
+++ b/src/DataDefinition/Palette/PaletteCollectionInterface.php
@@ -97,7 +97,7 @@ public function hasPalette(PaletteInterface $palette);
      *
      * @return PaletteInterface
      */
-    public function findPalette(ModelInterface $model = null, PropertyValueBagInterface $input = null);
+    public function findPalette(?ModelInterface $model = null, ?PropertyValueBagInterface $input = null);
 
     /**
      * Check if a palette for the given name exists in this collection.
diff --git a/src/DataDefinition/Palette/PaletteInterface.php b/src/DataDefinition/Palette/PaletteInterface.php
index 2a4673256..fb175e7c4 100644
--- a/src/DataDefinition/Palette/PaletteInterface.php
+++ b/src/DataDefinition/Palette/PaletteInterface.php
@@ -50,32 +50,32 @@ public function getName();
     /**
      * Get all properties from all legends in this palette.
      *
-     * @param ModelInterface|null $model If given, selectors will be evaluated depending on the model.
-     * @param PropertyValueBag    $input If given, selectors will be evaluated depending on the input data.
+     * @param ModelInterface|null   $model If given, selectors will be evaluated depending on the model.
+     * @param PropertyValueBag|null $input If given, selectors will be evaluated depending on the input data.
      *
      * @return PropertyInterface[]
      */
-    public function getProperties(ModelInterface $model = null, PropertyValueBag $input = null);
+    public function getProperties(?ModelInterface $model = null, ?PropertyValueBag $input = null);
 
     /**
      * Get all properties from all legends in this palette that are visible.
      *
-     * @param ModelInterface|null $model If given, selectors will be evaluated depending on the model.
-     * @param PropertyValueBag    $input If given, selectors will be evaluated depending on the input data.
+     * @param ModelInterface|null   $model If given, selectors will be evaluated depending on the model.
+     * @param PropertyValueBag|null $input If given, selectors will be evaluated depending on the input data.
      *
      * @return PropertyInterface[]
      */
-    public function getVisibleProperties(ModelInterface $model = null, PropertyValueBag $input = null);
+    public function getVisibleProperties(?ModelInterface $model = null, ?PropertyValueBag $input = null);
 
     /**
      * Get all properties from all legends in this palette that are editable.
      *
-     * @param ModelInterface|null $model If given, selectors will be evaluated depending on the model.
-     * @param PropertyValueBag    $input If given, selectors will be evaluated depending on the input data.
+     * @param ModelInterface|null   $model If given, selectors will be evaluated depending on the model.
+     * @param PropertyValueBag|null $input If given, selectors will be evaluated depending on the input data.
      *
      * @return PropertyInterface[]
      */
-    public function getEditableProperties(ModelInterface $model = null, PropertyValueBag $input = null);
+    public function getEditableProperties(?ModelInterface $model = null, ?PropertyValueBag $input = null);
 
     /**
      * Get a property by name from this palette.
@@ -108,11 +108,11 @@ public function setLegends(array $legends);
      * Add all legends to this palette.
      *
      * @param list<LegendInterface> $legends The legends.
-     * @param LegendInterface       $before  The legend before which the new legends shall be inserted (optional).
+     * @param LegendInterface|null  $before  The legend before which the new legends shall be inserted (optional).
      *
      * @return PaletteInterface
      */
-    public function addLegends(array $legends, LegendInterface $before = null);
+    public function addLegends(array $legends, ?LegendInterface $before = null);
 
     /**
      * Determine if a legend with the given name exists in this palette.
@@ -136,11 +136,11 @@ public function containsLegend(LegendInterface $legend);
      * Add a legend to this palette.
      *
      * @param LegendInterface $legend The legend to add.
-     * @param LegendInterface $before The legend before which the new legend shall be inserted (optional).
+     * @param LegendInterface|null $before The legend before which the new legend shall be inserted (optional).
      *
      * @return PaletteInterface
      */
-    public function addLegend(LegendInterface $legend, LegendInterface $before = null);
+    public function addLegend(LegendInterface $legend, ?LegendInterface $before = null);
 
     /**
      * Remove a legend from this palette.
@@ -176,7 +176,7 @@ public function getLegends();
      *
      * @return PaletteInterface
      */
-    public function setCondition(PaletteConditionInterface $condition = null);
+    public function setCondition(?PaletteConditionInterface $condition = null);
 
     /**
      * Get the condition bound to this palette.
diff --git a/src/DataDefinition/Palette/Property.php b/src/DataDefinition/Palette/Property.php
index 3b00cc9a2..d1dda68b2 100644
--- a/src/DataDefinition/Palette/Property.php
+++ b/src/DataDefinition/Palette/Property.php
@@ -85,9 +85,9 @@ public function getName()
      * {@inheritdoc}
      */
     public function isVisible(
-        ModelInterface $model = null,
-        PropertyValueBagInterface $input = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBagInterface $input = null,
+        ?LegendInterface $legend = null
     ) {
         if ($this->visibleCondition) {
             // We should have defined the interfaces back in 2013... :/
@@ -102,9 +102,9 @@ public function isVisible(
      * {@inheritdoc}
      */
     public function isEditable(
-        ModelInterface $model = null,
-        PropertyValueBagInterface $input = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBagInterface $input = null,
+        ?LegendInterface $legend = null
     ) {
         if ($this->editableCondition) {
             // We should have defined the interfaces back in 2013... :/
@@ -118,7 +118,7 @@ public function isEditable(
     /**
      * {@inheritdoc}
      */
-    public function setVisibleCondition(PropertyConditionInterface $condition = null)
+    public function setVisibleCondition(?PropertyConditionInterface $condition = null)
     {
         $this->visibleCondition = $condition;
 
@@ -136,7 +136,7 @@ public function getVisibleCondition()
     /**
      * {@inheritdoc}
      */
-    public function setEditableCondition(PropertyConditionInterface $condition = null)
+    public function setEditableCondition(?PropertyConditionInterface $condition = null)
     {
         $this->editableCondition = $condition;
 
diff --git a/src/DataDefinition/Palette/PropertyInterface.php b/src/DataDefinition/Palette/PropertyInterface.php
index e88b3bd25..8032d6904 100644
--- a/src/DataDefinition/Palette/PropertyInterface.php
+++ b/src/DataDefinition/Palette/PropertyInterface.php
@@ -62,9 +62,9 @@ public function getName();
      * @return bool
      */
     public function isVisible(
-        ModelInterface $model = null,
-        PropertyValueBagInterface $input = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBagInterface $input = null,
+        ?LegendInterface $legend = null
     );
 
     /**
@@ -77,24 +77,24 @@ public function isVisible(
      *                                               data.
      *                                               If no model and no input data is given, all properties will be
      *                                               returned, including sub palette properties.
-     * @param LegendInterface                $legend The legend the property is assigned to.
+     * @param LegendInterface|null           $legend The legend the property is assigned to.
      *
      * @return bool
      */
     public function isEditable(
-        ModelInterface $model = null,
-        PropertyValueBagInterface $input = null,
-        LegendInterface $legend = null
+        ?ModelInterface $model = null,
+        ?PropertyValueBagInterface $input = null,
+        ?LegendInterface $legend = null
     );
 
     /**
      * Set the visible condition for this property.
      *
-     * @param PropertyConditionInterface $condition The condition.
+     * @param PropertyConditionInterface|null $condition The condition.
      *
      * @return PropertyInterface
      */
-    public function setVisibleCondition(PropertyConditionInterface $condition = null);
+    public function setVisibleCondition(?PropertyConditionInterface $condition = null);
 
     /**
      * Get the visible condition for this property.
@@ -106,11 +106,11 @@ public function getVisibleCondition();
     /**
      * Set the editable condition for this property.
      *
-     * @param PropertyConditionInterface $condition The condition.
+     * @param PropertyConditionInterface|null $condition The condition.
      *
      * @return PropertyInterface
      */
-    public function setEditableCondition(PropertyConditionInterface $condition = null);
+    public function setEditableCondition(?PropertyConditionInterface $condition = null);
 
     /**
      * Get the editable condition for this property.
diff --git a/src/Event/EnforceModelRelationshipEvent.php b/src/Event/EnforceModelRelationshipEvent.php
index ceb23c608..9599b9006 100644
--- a/src/Event/EnforceModelRelationshipEvent.php
+++ b/src/Event/EnforceModelRelationshipEvent.php
@@ -53,8 +53,8 @@ class EnforceModelRelationshipEvent extends AbstractModelAwareEvent
     public function __construct(
         EnvironmentInterface $environment,
         ModelInterface $model,
-        ModelInterface $parentModel = null,
-        ModelInterface $rootModel = null
+        ?ModelInterface $parentModel = null,
+        ?ModelInterface $rootModel = null
     ) {
         parent::__construct($environment, $model);
 
diff --git a/src/Event/PostPersistModelEvent.php b/src/Event/PostPersistModelEvent.php
index 76f01f276..36996367c 100644
--- a/src/Event/PostPersistModelEvent.php
+++ b/src/Event/PostPersistModelEvent.php
@@ -48,7 +48,7 @@ class PostPersistModelEvent extends AbstractModelAwareEvent
     public function __construct(
         EnvironmentInterface $environment,
         ModelInterface $model,
-        ModelInterface $originalModel = null
+        ?ModelInterface $originalModel = null
     ) {
         parent::__construct($environment, $model);
 
diff --git a/src/Event/PrePersistModelEvent.php b/src/Event/PrePersistModelEvent.php
index 496cd9a80..a80ef5e2c 100644
--- a/src/Event/PrePersistModelEvent.php
+++ b/src/Event/PrePersistModelEvent.php
@@ -48,7 +48,7 @@ class PrePersistModelEvent extends AbstractModelAwareEvent
     public function __construct(
         EnvironmentInterface $environment,
         ModelInterface $model,
-        ModelInterface $originalModel = null
+        ?ModelInterface $originalModel = null
     ) {
         parent::__construct($environment, $model);
 
diff --git a/src/EventListener/StoreRefererListener.php b/src/EventListener/StoreRefererListener.php
index a14af5386..dcf69d510 100644
--- a/src/EventListener/StoreRefererListener.php
+++ b/src/EventListener/StoreRefererListener.php
@@ -84,6 +84,9 @@ public function __invoke(ResponseEvent $event): void
         $session   = $request->getSession();
         $key       = $request->query->has('popup') ? 'popupReferer' : 'referer';
         $refererId = $request->attributes->get('_contao_referer_id');
+        if (null === $refererId) {
+            return;
+        }
         $referers  = $this->prepareBackendReferer($refererId, $session->get($key));
         $ref       = (string) $request->query->get('ref', '');
 
diff --git a/src/Exception/DefinitionException.php b/src/Exception/DefinitionException.php
index 0e45aa04a..16bbc29ca 100644
--- a/src/Exception/DefinitionException.php
+++ b/src/Exception/DefinitionException.php
@@ -50,7 +50,7 @@ class DefinitionException extends DcGeneralRuntimeException
      * @param int             $code           The error code.
      * @param \Exception|null $previous       The previous exception.
      */
-    public function __construct($definitionName, $code = 0, \Exception $previous = null)
+    public function __construct($definitionName, $code = 0, ?\Exception $previous = null)
     {
         $this->name = $definitionName;
 
diff --git a/src/Factory/DcGeneralFactory.php b/src/Factory/DcGeneralFactory.php
index 519fd3dfb..6626093c2 100644
--- a/src/Factory/DcGeneralFactory.php
+++ b/src/Factory/DcGeneralFactory.php
@@ -58,7 +58,7 @@ class DcGeneralFactory implements DcGeneralFactoryInterface
      *
      * @param CacheInterface|null $cache The cache.
      */
-    public function __construct(CacheInterface $cache = null)
+    public function __construct(?CacheInterface $cache = null)
     {
         if (null === $cache) {
             // @codingStandardsIgnoreStart
@@ -299,7 +299,7 @@ public function getTranslator()
     /**
      * {@inheritdoc}
      */
-    public function setEnvironment(EnvironmentInterface $environment = null)
+    public function setEnvironment(?EnvironmentInterface $environment = null)
     {
         $this->environment = $environment;
 
@@ -317,7 +317,7 @@ public function getEnvironment()
     /**
      * {@inheritdoc}
      */
-    public function setDataContainer(ContainerInterface $dataContainer = null)
+    public function setDataContainer(?ContainerInterface $dataContainer = null)
     {
         $this->dataContainer = $dataContainer;
 
diff --git a/src/Panel/DefaultPanel.php b/src/Panel/DefaultPanel.php
index 2db61511d..ee8d82a67 100644
--- a/src/Panel/DefaultPanel.php
+++ b/src/Panel/DefaultPanel.php
@@ -95,7 +95,7 @@ public function getElement($elementName)
     /**
      * {@inheritdoc}
      */
-    public function initialize(ConfigInterface $config, PanelElementInterface $element = null)
+    public function initialize(ConfigInterface $config, ?PanelElementInterface $element = null)
     {
         /** @var PanelElementInterface $currentElement */
         foreach ($this as $currentElement) {
diff --git a/src/View/ActionHandler/AbstractPropertyVisibilityHandler.php b/src/View/ActionHandler/AbstractPropertyVisibilityHandler.php
index e826b6748..7fa724c63 100644
--- a/src/View/ActionHandler/AbstractPropertyVisibilityHandler.php
+++ b/src/View/ActionHandler/AbstractPropertyVisibilityHandler.php
@@ -788,7 +788,7 @@ private function useIntersectValue(
         $intersectPropertyName,
         array $legendPropertyNames,
         EnvironmentInterface $environment,
-        PaletteInterface $defaultPalette = null
+        ?PaletteInterface $defaultPalette = null
     ): bool {
         $propertiesDefinition = $this->getDataDefinition($environment)->getPropertiesDefinition();
         $useIntersectValue    = (bool) $defaultPalette;
@@ -894,7 +894,7 @@ private function intersectModelSetParentId(ModelInterface $intersectModel, Envir
     private function getLegendPropertyNames(
         ModelInterface $intersectModel,
         EnvironmentInterface $environment,
-        PaletteInterface &$defaultPalette = null
+        ?PaletteInterface &$defaultPalette = null
     ): array {
         $inputProvider      = $this->getInputProvider($environment);
         $palettesDefinition = $this->getDataDefinition($environment)->getPalettesDefinition();
diff --git a/tests/Clipboard/MockedAbstractItem.php b/tests/Clipboard/MockedAbstractItem.php
index ee4e14ad0..29cbbf111 100644
--- a/tests/Clipboard/MockedAbstractItem.php
+++ b/tests/Clipboard/MockedAbstractItem.php
@@ -54,7 +54,7 @@ class MockedAbstractItem extends AbstractItem
      *
      * @SuppressWarnings(PHPMD.LongVariable)
      */
-    public function __construct($action, ModelIdInterface $parentId = null, $modelIdOrProviderName = null)
+    public function __construct($action, ?ModelIdInterface $parentId = null, $modelIdOrProviderName = null)
     {
         parent::__construct($action, $parentId);