public function delete_custom_group(){
    global $wpdb;

    //checking the nonce
    check_admin_referer('delete_custom_group');

    if( isset($_GET['custom_group_id']) ){
      $id = (int)$_GET['custom_group_id'];
      if( is_int($id) ){
        $group = $this->get_group($id);
        $sql = $wpdb->prepare( "DELETE FROM ".MF_TABLE_CUSTOM_GROUPS." WHERE id = %d",$id );
        $wpdb->query($sql);

        $sql_fields = $wpdb->prepare( "DELETE FROM ".MF_TABLE_CUSTOM_FIELDS." WHERE custom_group_id = %d",$id );
        $wpdb->query($sql_fields);

        //ToDo: poner mensaje de que se borro correctamente
        $this->mf_redirect('mf_custom_fields','fields_list',array('message' => 'success','post_type' => $group['post_type']));

      }
    }
  }