[6.1] When i changing user group permission getting errors

[Jacara]

First i changed some group assignment for a user. After user login to server by web client then getting errors:

Server Traceback (most recent call last):
  File "/home/user/openerp/addons/web/common/http.py", line 592, in send
    result = openerp.netsvc.dispatch_rpc(service_name, method, args)
  File "/home/user/openerp/netsvc.py", line 360, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
  File "/home/user/openerp/service/web_services.py", line 572, in dispatch
    res = fn(db, uid, *params)
  File "/home/user/openerp/osv/osv.py", line 167, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
  File "/home/user/openerp/osv/osv.py", line 121, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/user/openerp/osv/osv.py", line 176, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/user/openerp/addons/audittrail/audittrail.py", line 495, in execute_cr
    return fct_src(cr, uid, model, method, *args)
  File "/home/user/openerp/osv/osv.py", line 164, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/user/openerp/addons/base/ir/ir_ui_menu.py", line 123, in search
    result = self._filter_visible_menus(cr, uid, ids, context=context)
  File "/home/user/openerp/addons/base/ir/ir_ui_menu.py", line 99, in _filter_visible_menus
    if not menu.child_id:
  File "/home/user/openerp/osv/orm.py", line 476, in __getattr__
    return self[name]
  File "/home/user/openerp/osv/orm.py", line 391, in __getitem__
    field_values = self._table.read(self._cr, self._uid, ids, field_names, context=self._context, load="_classic_write")
  File "/home/user/openerp/osv/orm.py", line 3384, in read
    result = self._read_flat(cr, user, select, fields, context, load)
  File "/home/user/openerp/osv/orm.py", line 3507, in _read_flat
    res2 = self._columns[f].get(cr, self, ids, f, user, context=context, values=res)
  File "/home/user/openerp/osv/fields.py", line 551, in get
    ids2 = obj.pool.get(self._obj).search(cr, user, self._domain + [(self._fields_id, 'in', ids)], limit=self._limit, context=context)
  File "/home/user/openerp/addons/base/ir/ir_ui_menu.py", line 123, in search
    result = self._filter_visible_menus(cr, uid, ids, context=context)
  File "/home/user/openerp/addons/base/ir/ir_ui_menu.py", line 99, in _filter_visible_menus
    if not menu.child_id:
  File "/home/user/openerp/osv/orm.py", line 476, in __getattr__
    return self[name]
  File "/home/user/openerp/osv/orm.py", line 391, in __getitem__
    field_values = self._table.read(self._cr, self._uid, ids, field_names, context=self._context, load="_classic_write")
  File "/home/user/openerp/osv/orm.py", line 3384, in read
    result = self._read_flat(cr, user, select, fields, context, load)
  File "/home/user/openerp/osv/orm.py", line 3507, in _read_flat
    res2 = self._columns[f].get(cr, self, ids, f, user, context=context, values=res)
  File "/home/user/openerp/osv/fields.py", line 551, in get
    ids2 = obj.pool.get(self._obj).search(cr, user, self._domain + [(self._fields_id, 'in', ids)], limit=self._limit, context=context)
  File "/home/user/openerp/addons/base/ir/ir_ui_menu.py", line 123, in search
    result = self._filter_visible_menus(cr, uid, ids, context=context)
  File "/home/user/openerp/addons/base/ir/ir_ui_menu.py", line 95, in _filter_visible_menus
    if not modelaccess.check(cr, uid, data[field], 'read', False):
  File "/home/user/openerp/tools/cache.py", line 18, in lookup
    r = self.lookup(self2, cr, *args)
  File "/home/user/openerp/tools/cache.py", line 46, in lookup
    value = d[args] = self.method(self2, cr, *args)
  File "/home/user/openerp/addons/base/ir/ir_model.py", line 509, in check
    if self.pool.get(model_name).is_transient():
AttributeError: 'NoneType' object has no attribute 'is_transient'

I user delete browser cache then errors will be solved. But i think it's bug.
User can not understand this error messages. They need to show understandable messages such as "Please delete your browser caches and re-login".