sale_crm : After deleting event, event invitations is not working

Install sale_crm

Sale > configuration > Calendar > Events

Steps:
          -Create on event and invite people
          -whoever peoples are invited will be shown in event invitations menu
          -now delete event
          -event invitation menu will not open and generate following Trace

Traceback (most recent call last):
  File "/home/han/rdtool/server/trunk/openerp/osv/osv.py", line 120, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/han/rdtool/server/trunk/openerp/osv/osv.py", line 172, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/han/rdtool/server/trunk/openerp/osv/osv.py", line 163, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/han/rdtool/server/trunk/openerp/osv/orm.py", line 3348, in read
    result = self._read_flat(cr, user, select, fields, context, load)
  File "/home/han/rdtool/server/trunk/openerp/osv/orm.py", line 3459, in _read_flat
    res2 = self._columns[val[0]].get(cr, self, ids, val, user, context=context, values=res)
  File "/home/han/rdtool/server/trunk/openerp/osv/fields.py", line 1034, in get
    result = self._fnct(obj, cr, uid, ids, name, self._arg, context)
  File "/home/han/rdtool/addons/trunk/base_calendar/base_calendar.py", line 286, in _compute_data
    result[id][name] = (attdata.ref.user_id.id, attdata.ref.user_id.name)
  File "/home/han/rdtool/server/trunk/openerp/osv/orm.py", line 478, in __getattr__
    raise AttributeError(e)
AttributeError: 4