code: def existing_choice_control(self):
          
          self.current.task_data['existing'] = False
          self.current.task_data['msg'] = _(u"You should choose just one channel to do operation.")
          keys, names = self.return_selected_form_items(self.input['form']['ChannelList'])
          if len(keys) == 1:
              self.current.task_data['existing'] = True
              self.current.task_data['target_channel_key'] = keys[0]
docs: It controls errors. It generates an error message
          if zero or more than one channels are selected.