[Mac] Fix crash when checking if a widget has a native parent

Xwt.Mac/Xwt.Mac/MacEngine.cs

@@ -124,6 +124,7 @@ namespace Xwt.Mac
 			RegisterBackend <Xwt.Backends.IPasswordEntryBackend, PasswordEntryBackend> ();
 			RegisterBackend <Xwt.Backends.IWebViewBackend, WebViewBackend> ();
 			RegisterBackend <Xwt.Backends.ISaveFileDialogBackend, SaveFileDialogBackend> ();
+			RegisterBackend <Xwt.Backends.IColorPickerBackend, ColorPickerBackend> ();
 		}
 
 		public override void RunApplication ()
@@ -194,7 +195,10 @@ namespace Xwt.Mac
 
 		public override bool HasNativeParent (Widget w)
 		{
-			ViewBackend wb = (ViewBackend)Toolkit.GetBackend (w);
+			var b = (IWidgetBackend) Toolkit.GetBackend (w);
+			if (b is XwtWidgetBackend)
+				b = ((XwtWidgetBackend)b).NativeBackend;
+			ViewBackend wb = (ViewBackend)b;
 			return wb.Widget.Superview != null;
 		}
 		

Xwt/Xwt.Backends/XwtWidgetBackend.cs

@@ -117,6 +117,10 @@ namespace Xwt.Backends
 				Font = new Xwt.Drawing.Font (value);
 			}
 		}
+
+		public IWidgetBackend NativeBackend {
+			get { return BackendHost.Backend; }
+		}
 		
 		object IWidgetBackend.NativeWidget {
 			get { return BackendHost.Backend.NativeWidget; }