fix(leak): Ensure that ItemCollection is unregistering from ItemSource

This change also moves the null check at the first position for performance

src/Uno.UI/UI/Xaml/Controls/ItemCollection.cs

@@ -155,6 +155,7 @@ internal void SetItemsSource(IEnumerable itemsSource)
 			if (itemsSource == null)
 			{
 				_itemsSource = null;
+				ObserveCollectionChanged(null);
 			}
 			else
 			{
@@ -182,7 +183,12 @@ internal void SetItemsSource(IEnumerable itemsSource)
 
 		private void ObserveCollectionChanged(object itemsSource)
 		{
-			if (itemsSource is INotifyCollectionChanged existingObservable)
+			if (itemsSource is null)
+			{
+				// fast path for null
+				_itemsSourceCollectionChangeDisposable.Disposable = null;
+			}
+			else if (itemsSource is INotifyCollectionChanged existingObservable)
 			{
 				// This is a workaround for a bug with EventRegistrationTokenTable on Xamarin, where subscribing/unsubscribing to a class method directly won't 
 				// remove the handler.

src/Uno.UI/UI/Xaml/Controls/ItemsControl/ItemsControl.cs

@@ -660,8 +660,12 @@ internal void ObserveCollectionChanged()
 		{
 			var unwrappedSource = UnwrapItemsSource();
 
+			if(unwrappedSource is null)
+			{
+				_notifyCollectionChanged.Disposable = null;
+			}
 			//Subscribe to changes on grouped source that is an observable collection
-			if (unwrappedSource is CollectionView collectionView && collectionView.CollectionGroups != null && collectionView.InnerCollection is INotifyCollectionChanged observableGroupedSource)
+			else if (unwrappedSource is CollectionView collectionView && collectionView.CollectionGroups != null && collectionView.InnerCollection is INotifyCollectionChanged observableGroupedSource)
 			{
 				// This is a workaround for a bug with EventRegistrationTokenTable on Xamarin, where subscribing/unsubscribing to a class method directly won't 
 				// remove the handler.