fix: Handle collection properties in custom markup extensions

Co-authored-by: nickrandolph <1614057+nickrandolph@users.noreply.github.com>

Author: Copilot

src/Uno.UI/UI/Xaml/Markup/Reader/XamlObjectBuilder.cs

@@ -1049,7 +1049,38 @@ private void ProcessCustomMarkupExtension(object instance, object? rootInstance,
 				{
 					if (propertyInfo != null)
 					{
-						GetPropertySetter(propertyInfo).Invoke(instance, new[] { value });
+						// Check if this is an initialized collection property (like Panel.Children)
+						if (TypeResolver.IsInitializedCollection(propertyInfo))
+						{
+							if (propertyInfo.GetMethod == null)
+							{
+								throw new InvalidOperationException($"The property {propertyInfo} does not provide a getter (Line {member.LineNumber}:{member.LinePosition}");
+							}
+
+							var propertyInstance = propertyInfo.GetMethod.Invoke(instance, null);
+
+							if (propertyInstance != null)
+							{
+								// Add the value returned by the markup extension to the collection
+								var addMethod = propertyInstance.GetType().GetMethod("Add");
+								if (addMethod != null)
+								{
+									addMethod.Invoke(propertyInstance, new[] { value });
+								}
+								else
+								{
+									throw new InvalidOperationException($"Unable to find Add method for collection property [{propertyInfo}]");
+								}
+							}
+							else
+							{
+								throw new InvalidOperationException($"The property {propertyInfo} getter did not provide a value (Line {member.LineNumber}:{member.LinePosition}");
+							}
+						}
+						else
+						{
+							GetPropertySetter(propertyInfo).Invoke(instance, new[] { value });
+						}
 					}
 					else if (TypeResolver.FindDependencyProperty(member) is { } dependencyProperty &&
 							 instance is DependencyObject dependencyObject)