Error dialog about missing config key "top_pane" at every startup

[David Gowers]

Each time zim starts up, it shows an error dialog. Once the dialog is closed, zim appears to work normally.
The error seems to be about a missing config variable.

Revision 825 is not the first that this happens with. I've repeatedly updated in the course of.. approximately the last month.. in the hope that this bug will go away.

No manual editing of config file was involved.

Contents of the error dialog:

This is zim 0.65
Platform: posix
Locale: en_AU UTF-8
FS encoding: UTF-8
Python: (2, 7, 11, 'final', 0)
Gtk: (2, 24, 30)
Pygtk: (2, 24, 0)
Zim revision is:
  branch: zim
  revision: 825 <email address hidden>
  date: 2016-02-22 21:24:57 +0100

======= Traceback =======
  File "/usr/lib/python2.7/site-packages/zim/actions.py", line 240, in do_activate
    self.__get__(instance, instance.__class__)()
  File "/usr/lib/python2.7/site-packages/zim/actions.py", line 211, in func
    self.func(instance, active)
  File "/usr/lib/python2.7/site-packages/zim/gui/__init__.py", line 2325, in toggle_panes
    Window.save_uistate(self)
  File "/usr/lib/python2.7/site-packages/zim/gui/widgets.py", line 2625, in save_uistate
    self.uistate[key] = self.get_pane_state(key)
  File "/usr/lib/python2.7/site-packages/zim/config/dicts.py", line 531, in __setitem__
    raise KeyError('Config key "%s" has not been defined' % k)
KeyError: 'Config key "top_pane" has not been defined'