code: def subscriber_choice_control(self):
          
          self.current.task_data['option'] = None
          self.current.task_data['chosen_subscribers'], names = self.return_selected_form_items(
              self.input['form']['SubscriberList'])
          self.current.task_data[
              'msg'] = "You should choose at least one subscriber for migration operation."
          if self.current.task_data['chosen_subscribers']:
              self.current.task_data['option'] = self.input['cmd']
              del self.current.task_data['msg']
docs: It controls subscribers choice and generates
          error message if there is a non-choice.