Add search to front page menu in Sela

Hi. I am modifying the header to add a search field. I have been able to add it and move it around to different positions. But none of them are quite right.

I would really like to have it at the end of the front page navigation menu. Floating inline with it immediately after as if it were the last menu item. I found this code in another forum to add search to the Nav menu. But when I add it to functions.php WordPress goes crazy and even the wp-admin site displays incorrectly.

Any idea how to make this code work with Sela in the current WordPress version?

add_filter('wp_nav_menu_items','add_search_box_to_menu', 10, 2);
function add_search_box_to_menu( $items, $args ) {
    if( $args->theme_location == 'primary' )
        return $items.get_search_form();

    return $items;
}