TypeError: coercing to Unicode: need string or buffer, NoneType found (setting page)

[rt@…]

After a upgrade from Python 2.6 to Python 2.7 i've this error message on my "settings page".

Datei "/usr/lib/python2.7/dist-packages/trac/web/main.py", Zeile 522, in _dispatch_request
  dispatcher.dispatch(req)
Datei "/usr/lib/python2.7/dist-packages/trac/web/main.py", Zeile 243, in dispatch
  resp = chosen_handler.process_request(req)
Datei "/usr/lib/python2.7/dist-packages/trac/prefs/web_ui.py", Zeile 69, in process_request
  for name, label in provider.get_preference_panels(req) or []:
Datei "/usr/lib/pymodules/python2.7/acct_mgr/web_ui.py", Zeile 138, in get_preference_panels
  user_store = AccountManager(self.env).find_user_store(req.authname)
Datei "/usr/lib/pymodules/python2.7/acct_mgr/api.py", Zeile 238, in find_user_store
  userlist = store.get_users()
Datei "/usr/lib/pymodules/python2.7/acct_mgr/htfile.py", Zeile 38, in get_users
  if not os.path.exists(filename):
Datei "/usr/lib/python2.7/genericpath.py", Zeile 18, in exists
  os.stat(path)

System informationen:

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_5) AppleWebKit/536.26.17 (KHTML, like Gecko) Version/6.0.2 Safari/536.26.17

Trac	0.12.3
Babel	0.9.6
Docutils	0.7
Genshi	0.6
Mercurial	2.2.2
mod_wsgi	3.3 (WSGIProcessGroup WSGIApplicationGroup trac.in.adspert.de|)
Pygments	1.3.1
pysqlite	2.6.0
Python	2.7.3rc2 (default, Apr 22 2012, 22:50:19) [GCC 4.6.3]
pytz	2010b
setuptools	0.6
SQLite	3.7.3
Subversion	1.6.17 (r1128011)
jQuery	1.4.2

[Jun Omae]

PluginIssue (​th:AccountManagerPlugin)