Fix PHPStan error calling getFlattenedSubsiteHierarchy method #156

Author: stephen-cox

src/Plugin/PreviewLinkAutopopulate/Subsites.php

@@ -3,6 +3,7 @@
 namespace Drupal\localgov_subsites\Plugin\PreviewLinkAutopopulate;
 
 use Drupal\localgov_subsites\Plugin\Block\SubsitesHierarchyTrait;
+use Drupal\node\NodeInterface;
 use Drupal\preview_link\PreviewLinkAutopopulatePluginBase;
 
 /**
@@ -28,6 +29,7 @@ class Subsites extends PreviewLinkAutopopulatePluginBase {
    * {@inheritdoc}
    */
   public function getPreviewEntities(): array {
+    assert($this->entity instanceof NodeInterface);
     return $this->getFlattenedSubsiteHierarchy($this->entity);
   }