fix: validate for null

Co-authored-by: David <github.site@torick.net>

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

@@ -259,7 +259,7 @@ private static void OnTitlePropertyChanged(DependencyObject dependencyObject, De
 		private void OnSelectedItemPropertyChanged(object oldValue, object newValue)
 		{
 			var removedItems = oldValue == null ? new object[0] : new [] { oldValue };
-			var addedItems = new [] { newValue };
+			var addedItems = newValue == null ? new object[0] : new [] { newValue };
 
 			OnSelectedItemChangedPartial(oldValue, newValue);