Add anchor links as submenu

/**
 * Custom Main Menu Walker
 */
class Main_Nav_Walker extends Walker_Nav_Menu {
    function check_current($classes) {
        return preg_match('/(current[-_])|active|dropdown/', $classes);
    }
    function start_lvl(&$output, $depth = 0, $args = array()) {
        $output .= "\n<ul class=\"dropdown-menu\">\n";
    }
    function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {
        parent::start_el($item_html, $item, $depth, $args);
        if ($item->is_dropdown && ($depth === 0)) {
            $item_html = str_replace('<a', '<a class="dropdown-toggle" data-toggle="dropdown" data-target="#"', $item_html);
            $item_html = str_replace('</a>', ' <b class="caret"></b></a>', $item_html);
        } elseif (stristr($item_html, 'li class="divider')) {
            $item_html = preg_replace('/<a[^>]*>.*?<\/a>/iU', '', $item_html);
        } elseif (stristr($item_html, 'li class="dropdown-header')) {
            $item_html = preg_replace('/<a[^>]*>(.*)<\/a>/iU', '$1', $item_html);
        }
        $item_html = apply_filters('roots_wp_nav_menu_item', $item_html);
        $output .= $item_html;
    }
    function display_element($element, &$children_elements, $max_depth, $depth = 0, $args, &$output) {
        // Add anchors as children elements
        if ($depth == 0){
            $elementid  = $element->ID;
            $objectid   = $element->object_id;
            // Get subitems
            $subitems = get_field('sub_items', $objectid);
            if ($subitems) {
                // Construct child array from anchors
                $children_elements = '';
                foreach ($subitems as $wp_post) {
                    $item_title     = $wp_post['sub_item_title'];
                    $item_anchor    = sanitize_title($item_title);
                    // Create new Array for each item
                    $children_elements[$elementid][] = array(
                        'ID'                    => 124,
                        'post_title'                => $item_title,
                        'post_name'                 => $item_anchor,
                        'post_type'                 => 'nav_menu_item',
                        'db_id'                 => 124,
                        'menu_item_parent'          => $elementid,
                        'object_id'                 => 124,
                        'object'                => 'custom',
                        'type'                  => 'custom',
                        'type_label'                => 'Anchor',
                        'title'                 => $item_title,
                        'url'                   => '#'.$item_anchor,
                        'target'                => '',
                        'attr_title'                => $item_title
                    );
                }
                //print_r($children_elements);
            }
        }

        $element->is_dropdown = ((!empty($children_elements[$element->ID]) && (($depth + 1) < $max_depth || ($max_depth === 0))));
        if ($element->is_dropdown) {
            $element->classes[] = 'dropdown';
        }
        parent::display_element($element, $children_elements, $max_depth, $depth, $args, $output);
    }
}