php - Object of class GBL_Extras could not be be converted to string -

i try plugin wordpress working stand in front of wall.


problem if call class gbl_extras error occured:

catchable fatal error: object of class gbl_extras not converted string in /wp-includes/functions.php on line 1592

don't figure out why happen. if delete add_submenu_page methods class render correctly, didn't mistake there.
if read many other articles same problems can't adapt problem error project.

there can bring little bit light in darkness?

my code:

<?php        // don't load directly if ( ! defined( 'abspath' ) ) {     die( '-1' ); }  if ( ! class_exists( 'gbl_extras' ) ) {     class gbl_extras {           private static $_instance;          private function __construct() {             // add hooks             add_action( 'network_admin_menu', array( $this, 'init' ));         }          public static function getinstance() {             if ( ! ( self::$_instance instanceof self ) ) {                 self::$_instance = new self();             }              return self::$_instance;         }          public function init() {                         add_menu_page( 'extrawünsche', 'extrawünsche', 'manage_options', 'gbl_extras_settings');             add_submenu_page( 'gbl_extras_settings', 'alle extrawünsche', 'alle extrawünsche', 'manage_options', array( $this, 'backendrenderextras'));             add_submenu_page( 'gbl_extras_settings', 'kategorien', 'kategorien', 'manage_options', 'gbl_extras_categories', array( $this, 'backendrendercategories'));         }          public function backendrenderextras() {             ?>             <div class="wrap">                 <div id="icon-users" class="icon32"><br/></div>                 <h2>extrawünsche</h2>             </div>             <?php         }          public function backendrendercategories() {             ?>             <div class="wrap">                 <h2>kategorien</h2>             </div>             <?php         }     }     $gbl_manager = gbl_extras::getinstance(); }  ?> 

looks first add_submenu_page() call missing $menu_slug argument. compare end of 2 calls:

'manage_options', array( $this, 'backendrenderextras')); 

and

'manage_options', 'gbl_extras_categories', array( $this, 'backendrendercategories')); 

the second 1 looks correct.