in_array() expects parameter 2 to be array, null given

Hi! My site suddenly broke down. It was okay for one second, I didn’t edit any codes. And when I refreshed, this error occurred:

( ! ) warning: in_array() expects parameter 2 to be array, null given in c:\wamp\www\cameraandcheese\wp-content\themes\_strap-master\_strap-master\inc\functions-strap.php on line 18

I started with a starter theme I found called _strap-master which is based on _s and bootstrap. Below is the code. Can anyone please verify where the error is? Line 18 is if( !in_array(‘dropdown’, $sorted_menu_items[$parent_index]->classes) ) {

function bootstrap_menu_objects($sorted_menu_items, $args)
{
    if($args->theme_location == 'primary')
    {//
        $current = array('current-menu-ancestor', 'current-menu-item');
        $registry = array();
        foreach($sorted_menu_items as $i => $item) {
            $is_current = array_intersect( (array) $item->classes, $current );
            if ( !empty($is_current) ) $item->classes[] = 'active';
            $registry[$item->ID] = $i;
            if($item->menu_item_parent) {
                $parent_index = $registry[$item->menu_item_parent];
                if( !in_array('dropdown', $sorted_menu_items[$parent_index]->classes) ) {
                    $sorted_menu_items[$parent_index]->classes[] = 'dropdown';
                }
            }
        }
        //print_r($sorted_menu_items);print_r($args);exit;
    }
    return $sorted_menu_items;
}
add_filter( 'wp_nav_menu_objects', 'bootstrap_menu_objects', 10, 2 );