Ticket #59410: 59410.diff

menu.php

@@ -304 +304 @@
          * @param array $menu_order An ordered array of menu items.
          */
         $menu_order = apply_filters( 'menu_order', $menu_order );
-        $menu_order = array_flip( $menu_order );
+        if ( isset( $menu_order) ){
+                $menu_order = array_flip( $menu_order );
+        }
+        
 
         $default_menu_order = array_flip( $default_menu_order );