[Xwt] If the native window is null, return null

This is a more logical behaviour and prevents us from hitting silly
null reference exceptions later on in the apps lifespan.

This probably explains some mysterious null reference exceptions we've
been seeing for the last few weeks.

Xwt/Xwt.Engine/WidgetRegistry.cs

@@ -88,6 +88,8 @@ namespace Xwt.Engine
 		
 		public static WindowFrame WrapWindow (object nativeWindow)
 		{
+			if (nativeWindow == null)
+				return null;
 			return new NativeWindowFrame (Application.EngineBackend.GetBackendForWindow (nativeWindow));
 		}
 	}