Fixing referenced element for popup placement (#3344)

Possibly a fix for #3316

Author: Keboo

MaterialDesignThemes.Wpf/ComboBoxPopup.cs

@@ -219,7 +219,7 @@ private CustomPopupPlacement[] ComboBoxCustomPopupPlacementCallback(
 
         var visualAncestry = PlacementTarget.GetVisualAncestry().ToList();
 
-        var parent = visualAncestry.OfType<Panel>().ElementAt(1);
+        var parent = visualAncestry.OfType<Panel>().First();
         VisiblePlacementWidth = TreeHelper.GetVisibleWidth((FrameworkElement)PlacementTarget, parent, FlowDirection);
 
         var data = GetPositioningData(visualAncestry, popupSize, targetSize);
@@ -246,12 +246,10 @@ PositioningData GetPositioningData(IEnumerable<DependencyObject?> visualAncestry
         {
             var locationFromScreen = PlacementTarget.PointToScreen(new Point(0, 0));
 
-            var mainVisual = visualAncestry.OfType<Visual>().LastOrDefault();
-            if (mainVisual is null) throw new ArgumentException($"{nameof(visualAncestry)} must contains at least one {nameof(Visual)} control inside.");
-
-            var controlVisual = visualAncestry.OfType<Visual>().FirstOrDefault();
-            if (controlVisual == null) throw new ArgumentException($"{nameof(visualAncestry)} must contains at least one {nameof(Visual)} control inside.");
-
+            var mainVisual = visualAncestry.OfType<Visual>().LastOrDefault()
+                ?? throw new ArgumentException($"{nameof(visualAncestry)} must contains at least one {nameof(Visual)} control inside.");
+            var controlVisual = visualAncestry.OfType<Visual>().FirstOrDefault()
+                ?? throw new ArgumentException($"{nameof(visualAncestry)} must contains at least one {nameof(Visual)} control inside.");
             var screen = Screen.FromPoint(locationFromScreen);
             var screenWidth = (int)screen.Bounds.Width;
             var screenHeight = (int)screen.Bounds.Height;