Fix crashes for context menu on empty query

Fixed issue #434, thanks @pieper126

Wox/MainWindow.xaml.cs

@@ -895,6 +895,7 @@ namespace Wox
 
         private void ShowContextMenu(Result result)
         {
+            if (result == null) return;
             List<Result> results = PluginManager.GetContextMenusForPlugin(result);
             results.ForEach(o =>
             {