gradebook link on timed-out session throws an exception

This is when you click the link at the top of the page after a logged-in session has timed out. It should direct you to log back in.

Exception

  TypeError: ('Could not adapt', <zope.app.security.principalregistry.UnauthenticatedPrincipal object at 0x9a39dcc>, <InterfaceClass schooltool.person.interfaces.IPerson>)
Traceback

  File "/usr/lib/python2.5/site-packages/zope/publisher/publish.py", line 133, in publish
  result = publication.callObject(request, obj)
File "/usr/lib/python2.5/site-packages/zope/app/publication/zopepublication.py", line 167, in callObject
  return mapply(ob, request.getPositionalArguments(), request)
File "/usr/lib/python2.5/site-packages/zope/publisher/publish.py", line 108, in mapply
  return debug_call(obj, args)
Extra information: <zope.app.pagetemplate.simpleviewclass.SimpleViewClass from /usr/lib/python2.5/site-packages/schooltool/gradebook/browser/gradebook_startup.pt object at 0xc0e838c>
File "/usr/lib/python2.5/site-packages/zope/publisher/publish.py", line 114, in debug_call
  return obj(*args)
File "/usr/lib/python2.5/site-packages/zope/app/pagetemplate/simpleviewclass.py", line 44, in __call__
  return self.index(*args, **kw)
File "/usr/lib/python2.5/site-packages/zope/app/pagetemplate/viewpagetemplatefile.py", line 83, in __call__
  return self.im_func(im_self, *args, **kw)
File "/usr/lib/python2.5/site-packages/zope/app/pagetemplate/viewpagetemplatefile.py", line 51, in __call__
  sourceAnnotations=getattr(debug_flags, 'sourceAnnotations', 0),
File "/usr/lib/python2.5/site-packages/zope/pagetemplate/pagetemplate.py", line 115, in pt_render
  strictinsert=0, sourceAnnotations=sourceAnnotations)()
Template "/usr/lib/python2.5/site-packages/schooltool/gradebook/browser/gradebook_startup.pt"
File "/usr/lib/python2.5/site-packages/zope/tal/talinterpreter.py", line 271, in __call__
  self.interpret(self.program)
File "/usr/lib/python2.5/site-packages/zope/tal/talinterpreter.py", line 346, in interpret
  handlers[opcode](self, args)
File "/usr/lib/python2.5/site-packages/zope/tal/talinterpreter.py", line 854, in do_condition
  if not self.tal or self.engine.evaluateBoolean(condition):
File "/usr/lib/python2.5/site-packages/zope/tales/tales.py", line 701, in evaluateBoolean
  return not not self.evaluate(expr)
File "/usr/lib/python2.5/site-packages/zope/tales/tales.py", line 696, in evaluate
  return expression(self)
Template "/usr/lib/python2.5/site-packages/schooltool/gradebook/browser/gradebook_startup.pt", line 1, column 0
  Expression: <PathExpr standard:u'view/update'>
File "/usr/lib/python2.5/site-packages/zope/tales/expressions.py", line 217, in __call__
  return self._eval(econtext)
File "/usr/lib/python2.5/site-packages/zope/tales/expressions.py", line 211, in _eval
  return ob()
File "/usr/lib/python2.5/site-packages/schooltool/gradebook/browser/gradebook.py", line 60, in update
  self.person = IPerson(self.request.principal)