def selection_error_control(self, form_info):
        """
        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

        """
        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
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