Ticket #32781: 32781.diff

wp-includes/nav-menu-template.php

@@ -308 +308 @@
          *  - Otherwise, bail.
          */
         if ( ( !$menu || is_wp_error($menu) || ( isset($menu_items) && empty($menu_items) && !$args->theme_location ) )
-                && $args->fallback_cb && is_callable( $args->fallback_cb ) )
+                && isset( $args->fallback_cb ) && $args->fallback_cb && is_callable( $args->fallback_cb ) )
                         return call_user_func( $args->fallback_cb, (array) $args );
 
         if ( ! $menu || is_wp_error( $menu ) )