alacarte crashed with TypeError in on_item_tree_drag_data_get(): could not parse subscript as a tree path

This happens when I drag and drop menu items in the Menu Editor in Gnome Classic. Steps:

1) Right click the top bar in Gnome Classic environment and select Edit Menu
2) Open one of the trees of the menu, and drag one menu item from the right to the tree menu to the left. That means we move one menu item into another menu tree.
3) When we release the mouse, quickly get another menu item from the right and drag to the same menu tree, in which the first menu item has dropped in.
4) The items would be moved successfully, but apport comes up with this error.

ProblemType: Crash
DistroRelease: Ubuntu 11.10
Package: alacarte 0.13.2-2ubuntu3
ProcVersionSignature: Ubuntu 3.0.0-12.19-generic 3.0.4
Uname: Linux 3.0.0-12-generic x86_64
ApportVersion: 1.23-0ubuntu2
Architecture: amd64
Date: Tue Oct 4 11:02:06 2011
ExecutablePath: /usr/bin/alacarte
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
InterpreterPath: /usr/bin/python2.7
PackageArchitecture: all
ProcCmdline: /usr/bin/python -OOt /usr/bin/alacarte
ProcEnviron:
 LANGUAGE=en_HK:en
 PATH=(custom, no user)
 LANG=en_HK.UTF-8
 SHELL=/bin/bash
PythonArgs: ['/usr/bin/alacarte']
SourcePackage: alacarte
Title: alacarte crashed with TypeError in on_item_tree_drag_data_get(): could not parse subscript as a tree path
Traceback:
 Traceback (most recent call last):
   File "/usr/share/alacarte/Alacarte/MainWindow.py", line 499, in on_item_tree_drag_data_get
     self.drag_data = items[iter][3]
 TypeError: could not parse subscript as a tree path
UpgradeStatus: Upgraded to oneiric on 2011-08-26 (39 days ago)
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