<?php
    $menu_name = 'primary';
    $locations = get_nav_menu_locations();
    $menu = wp_get_nav_menu_object( $locations[ $menu_name ] );
    $menuitems = wp_get_nav_menu_items( $menu->term_id, array( 'order' => 'DESC' ) );
    $count = 0;
    $submenu = false;
    foreach( $menuitems as $item ):
        // set up title and url
        $title = $item->title;
        $link = $item->url;

        // item does not have a parent so menu_item_parent equals 0 (false)
        if ( !$item->menu_item_parent ):

        // save this id for later comparison with sub-menu items
        $parent_id = $item->ID; ?>

  <li class="nav-item">
      <a class="nav-link text-white" href="<?php echo $link; ?>"><?php echo $title; ?></a>  <?php endif; ?>
      <?php if ( $parent_id == $item->menu_item_parent ): ?>
        <?php if ( !$submenu ): $submenu = true; ?>
<ul class="navbar-nav ml-auto sub-menu">
 <?php endif; ?>

    <li class="nav-item">
            <a href="<?php echo $link; ?>" class="nav-link text-white"><?php echo $title; ?></a>
        </li>

            <?php if ( $menuitems[ $count + 1 ]->menu_item_parent != $parent_id && $submenu ): ?>
    </ul>
    <?php $submenu = false; endif; ?>

<?php endif; ?>
<?php if ( $menuitems[ $count + 1 ]->menu_item_parent != $parent_id ): ?>
</li>                           
<?php $submenu = false; endif; ?>

<?php $count++; endforeach; ?>