WordPress – wp_nav_menu() show a submenu only

// Usage:

$args = array(
    'menu'    => 'Menu Name',
    'submenu' => 'About Us',
);

wp_nav_menu( $args );

//Paste this in function.php

add_filter( 'wp_nav_menu_objects', 'submenu_limit', 10, 2 );

function submenu_limit( $items, $args ) {

    if ( empty($args->submenu) )
        return $items;

    $parent_id = array_pop( wp_filter_object_list( $items, array( 'title' => $args->submenu ), 'and', 'ID' ) );
    //use 'object_id' => $args->submenu if need ID
    $children  = submenu_get_children_ids( $parent_id, $items );

    foreach ( $items as $key => $item ) {

        if ( ! in_array( $item->ID, $children ) )
            unset($items[$key]);
    }

    return $items;
}

function submenu_get_children_ids( $id, $items ) {

    $ids = wp_filter_object_list( $items, array( 'menu_item_parent' => $id ), 'and', 'ID' );

    foreach ( $ids as $id ) {

        $ids = array_merge( $ids, submenu_get_children_ids( $id, $items ) );
    }

    return $ids;
}

Hi,
I am not sure why its not working with me..
Notice: Only variables should be passed by reference in /wp-content/themes/mytheme/functions.php on line 12

On this line:
$parent_id = array_pop( wp_filter_object_list( $items, array( 'title' => $args->submenu ), 'and', 'ID' ) );