code: def get_activities_for_objective(self, objective_id=None):
          
          if objective_id is None:
              raise NullArgument()
          # Should also check if objective_id exists?
          url_path = construct_url('activities',
                                   bank_id=self._catalog_idstr,
                                   obj_id=objective_id)
          return objects.ActivityList(self._get_request(url_path))
docs: Gets the activities for the given objective.
          In plenary mode, the returned list contains all of the
          activities mapped to the objective Id or an error results if an
          Id in the supplied list is not found or inaccessible. Otherwise,
          inaccessible Activities may be omitted from the list and may
          present the elements in any order including returning a unique
          set.
          arg:    objectiveId (osid.id.Id): Id of the Objective
          return: (osid.learning.ActivityList) - list of enrollments
          raise:  NotFound - objectiveId not found
          raise:  NullArgument - objectiveId is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          compliance: mandatory - This method is must be implemented.