AttributeError: 'NoneType' object has no attribute 'has_new_modules'

I've just upgraded our test instance to trunk of both server and web. I'm now hitting the following traceback when I point a browser at the web address:

500 Internal Server Error

The server encountered an unexpected condition which prevented it from fulfilling the request.

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/cherrypy/_cprequest.py", line 606, in respond
    cherrypy.response.body = self.handler()
  File "/usr/lib/pymodules/python2.6/cherrypy/_cpdispatch.py", line 25, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/usr/share/openerp-web/openobject/tools/_expose.py", line 190, in func_wrapper
    res = func(*args, **kw)
  File "/usr/share/openerp-web/openobject/controllers/_root.py", line 67, in default
    if obj.has_new_modules():
AttributeError: 'NoneType' object has no attribute 'has_new_modules'

I don't really know how to go about debugging this, and would appreciate some suggestions.

TIA!