Merge pull request #834 from dkjonas/ListBoxAssistException

Fix NullReferenceException in ListBoxAssist

Author: ButchersBoy

MaterialDesignThemes.Wpf/ListBoxAssist.cs

@@ -27,6 +27,8 @@ private static void ListBoxMouseButtonEvent(object sender, MouseButtonEventArgs
             var point = mouseButtonEventArgs.GetPosition(senderElement);
             var result = VisualTreeHelper.HitTest(senderElement, point);
 
+            if (result == null) return;
+
             ListBoxItem listBoxItem = null;
             Ripple ripple = null;
             foreach (var dependencyObject in result.VisualHit.GetVisualAncestry().TakeWhile(_ => listBoxItem == null))