docs: def get_activity_form_for_update(self, activity_id=None):
          """Gets the activity form for updating an existing activity.
          A new activity form should be requested for each update
          transaction.
          arg:    activityId (osid.id.Id): the Id of the Activity
          return: (osid.learning.ActivityForm) - the activity form
          raise:  NotFound - activityId is not found
          raise:  NullArgument - activityId is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method must be implemented.
  
          """
          if activity_id is None:
              raise NullArgument()
          try:
              url_path = construct_url('activities',
                                       bank_id=self._catalog_idstr,
                                       act_id=activity_id)
              activity = objects.Activity(self._get_request(url_path))
          except Exception:
              raise
          activity_form = objects.ActivityForm(activity._my_map)
          self._forms[activity_form.get_id().get_identifier()] = not UPDATED
          return activity_form