fix: TargetPropertyPath is not releasing its WeakReference properly

Author: jeromelaban

src/Uno.UI.Tests/Windows_UI_Xaml_Markup/XamlReaderTests/Given_XamlReader.cs

@@ -726,6 +726,13 @@ public void When_IList_TabView()
 			Assert.AreEqual(2, tabView1.TabItems.Count);
 		}
 
+		[TestMethod]
+		public void When_StateTrigger_PropertyPath()
+		{
+			var s = GetContent(nameof(When_StateTrigger_PropertyPath));
+			var r = Windows.UI.Xaml.Markup.XamlReader.Load(s) as UserControl;
+		}
+
 		private string GetContent(string testName)
 		{
 			var assembly = this.GetType().Assembly;

src/Uno.UI.Tests/Windows_UI_Xaml_Markup/XamlReaderTests/When_StateTrigger_PropertyPath.xamltest

@@ -0,0 +1,28 @@
+<UserControl
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+>
+  <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+	<VisualStateManager.VisualStateGroups>
+	  <VisualStateGroup>
+		<VisualState>
+		  <VisualState.StateTriggers>
+			<!--VisualState to be triggered when window width is >=720 effective pixels.-->
+			<StateTrigger IsActive="True"/>
+		  </VisualState.StateTriggers>
+		  <VisualState.Setters>
+			<Setter Target="myPanel.Orientation" Value="Horizontal"/>
+		  </VisualState.Setters>
+		</VisualState>
+	  </VisualStateGroup>
+	</VisualStateManager.VisualStateGroups>
+	<StackPanel x:Name="myPanel" Orientation="Vertical">
+	  <TextBlock Text="This is a block of text. It is text block 1. "
+						   Style="{ThemeResource BodyTextBlockStyle}"/>
+	  <TextBlock Text="This is a block of text. It is text block 2. "
+						   Style="{ThemeResource BodyTextBlockStyle}"/>
+	  <TextBlock Text="This is a block of text. It is text block 3. "
+						   Style="{ThemeResource BodyTextBlockStyle}"/>
+	</StackPanel>
+  </Grid>
+</UserControl>

src/Uno.UI/UI/Xaml/TargetPropertyPath.cs

@@ -10,22 +10,31 @@ namespace Windows.UI.Xaml
 {
 	public sealed partial class TargetPropertyPath
 	{
+#if UNO_HAS_UIELEMENT_IMPLICIT_PINNING
 		private ManagedWeakReference? _targetRef;
+#endif
 
 		public object? Target
 		{
+#if UNO_HAS_UIELEMENT_IMPLICIT_PINNING
 			get => _targetRef?.Target;
 			set
 			{
 				if (_targetRef != null)
 				{
 					WeakReferencePool.ReturnWeakReference(this, _targetRef);
+					_targetRef = null;
 				}
-				else
+
+				if (!(value is null))
 				{
 					_targetRef = WeakReferencePool.RentWeakReference(this, value);
 				}
 			}
+#else
+			get;
+			set;
+#endif
 		}
 
 		public PropertyPath? Path