bugfix timespan editor on mediaelemnt when video is playing

(PropertyNode was ambigious as the same timestamp read twice differs)

Author: jogibear9988

WpfDesign.Designer/Project/PropertyGrid/Editors/TimeSpanEditor.xaml.cs

@@ -58,7 +58,11 @@ void NumberEditor_DataContextChanged(object sender, DependencyPropertyChangedEve
 			if (PropertyNode == null)
 				return;
 
-			var value = (TimeSpan) PropertyNode.DesignerValue;
+            var designerValue = PropertyNode.DesignerValue;
+            if (designerValue == null)
+                return;
+
+            var value = (TimeSpan)designerValue;
 
 
 			if (value < TimeSpan.Zero)