Problem moving search box to navbar menu

Hi, I have this code in functions.php of this site:

add_filter('i','add_search_box', 10, 2);
function add_search_box($items, $args) {

        ob_start();
        get_search_form();
        $searchform = ob_get_contents();
        ob_end_clean();

        $items .= '<li >' . $searchform . '</li>';

    return $items;
}

Everything seemed fine, searchbox was in the navbar but, suddenly, disappeared mysteriously. I don’t know why.

I’ll appreciate any clue about what and where to investigate.
Thank you in advance.