Fix a Null Ref Exception in Thumbnail View

(happend for Example on wrong XAML Code)

src/AddIns/DisplayBindings/WpfDesign/WpfDesign.Designer/Project/ThumbnailView/ThumbnailView.cs

@@ -113,9 +113,16 @@ namespace ICSharpCode.WpfDesign.Designer.ThumbnailView
 		}
 
 		private void InvalidateScale(out double scale, out double xOffset, out double yOffset)
+		{
+			scale = 1;
+			xOffset = 0;
+			yOffset = 0;
+			
+			if (this.DesignSurface.DesignContext != null)
 			{
 				var designedElement = this.DesignSurface.DesignContext.RootItem.Component as FrameworkElement;
 				
+				
 				var fac1 = designedElement.ActualWidth / zoomCanvas.ActualWidth;
 				var fac2 = designedElement.ActualHeight / zoomCanvas.ActualHeight;
 				
@@ -147,6 +154,7 @@ namespace ICSharpCode.WpfDesign.Designer.ThumbnailView
 				xOffset += (x - scale * w) / 2;
 				yOffset += (y - scale * h) / 2;
 			}
+		}
 
 		public event PropertyChangedEventHandler PropertyChanged;
 		protected virtual void OnPropertyChanged(string propertyName)