code: def selection_error_control(self, form_info):
          
          keys, names = self.return_selected_form_items(form_info['ChannelList'])
          chosen_channels_number = len(keys)
  
          if form_info['new_channel'] and chosen_channels_number < 2:
              return False, _(
                  u"You should choose at least two channel to merge operation at a new channel.")
          elif form_info['existing_channel'] and chosen_channels_number == 0:
              return False, _(
                  u"You should choose at least one channel to merge operation with existing channel.")
          elif form_info['find_chosen_channel'] and chosen_channels_number != 1:
              return False, _(u"You should choose one channel for split operation.")
  
          return True, None
docs: It controls the selection from the form according
          to the operations, and returns an error message
          if it does not comply with the rules.
  
          Args:
              form_info: Channel or subscriber form from the user
  
          Returns: True or False
                   error message