Drupal 8 Menu tree

/**
 * Override or insert variables into the page template.
 */
function gratis_preprocess_page(&$vars, $hook) {
// main menu tree with sub-links.
  if (!empty($vars['main_menu'])) {
    $menu_tree = \Drupal::menuTree();
    $menu_enabled = Drupal::moduleHandler()->moduleExists('menu');
    $config = Drupal::config('menu.settings');
    $menu_name = $menu_enabled ? $config->get('main_links') : 'main';
    $parameters = $menu_tree->getCurrentRouteMenuTreeParameters($menu_name);
    $parameters->setMaxDepth($level + 1);
    $tree = $menu_tree->load($menu_name, $parameters);
    $manipulators = array(
      array('callable' => 'menu.default_tree_manipulators:checkAccess'),
      array('callable' => 'menu.default_tree_manipulators:generateIndexAndSort'),
      array(
        'callable' => 'menu.default_tree_manipulators:extractSubtreeOfActiveTrail',
        'args' => array($level)
      ),
    );
    $tree = $menu_tree->transform($tree, $manipulators);
    return $menu_tree->build($tree);
    $vars['main_menu']['#attributes']['id'] = 'main-menu-links';
    $vars['main_menu']['#attributes']['class'] = array('links', 'clearfix');
    // set the theme menu function for use below.
    $vars['main_menu']['#theme_wrappers'] = array('menu_tree__primary');
  }
}

/**
 * Theme wrapper function for the primary menu links.
 */
function gratis_menu_tree__primary(&$vars) {
  return '<ul class="toggle-menu"></ul>';
}

// Then in page.html.twig, we render the menu

    {% if main_menu %}
     {{ main_menu }}
   {% endif %}