fix arbitrary content being dragged and dropped causing errors

Fixes #2630

gui/builtinAdditionPanes/commandView.py

@@ -56,6 +56,10 @@ class CommandViewDrop(wx.DropTarget):
     def OnData(self, x, y, t):
         if self.GetData():
             dragged_data = DragDropHelper.data
+
+            if dragged_data is None:
+                return t
+
             data = dragged_data.split(':')
             self.dropFn(x, y, data)
         return t