--- a/editors/ConfTreeNodeEditor.py	Thu Sep 26 20:47:36 2013 +0900
+++ b/editors/ConfTreeNodeEditor.py	Fri Sep 27 09:32:39 2013 +0900
@@ -496,6 +496,8 @@
     def GetChoiceCallBackFunction(self, choicectrl, path):
         def OnChoiceChanged(event):
             res = self.SetConfNodeParamsAttribute(path, choicectrl.GetStringSelection())
+            if res is None:
+                res = ""
             choicectrl.SetStringSelection(res)
             event.Skip()
         return OnChoiceChanged