fix: WPF TransitioningContentControl can crash

Author: glennawatson

src/ReactiveUI.Wpf/TransitioningContentControl.cs

@@ -221,7 +221,7 @@ protected override void OnContentChanged(object oldContent, object newContent)
 
         private static RenderTargetBitmap GetRenderTargetBitmapFromUiElement(UIElement uiElement)
         {
-            if (uiElement.RenderSize.Height == 0)
+            if (uiElement.RenderSize.Height == 0 || uiElement.RenderSize.Width == 0)
             {
                 return default!;
             }