Wordpress - Category Sub Menu Walker

functions.php

class Walker_Add_Myterms extends Walker_Nav_Menu
{
  public function end_el(&$output, $item, $depth = 1, $args = array())
  {
    if ($item->object === 'category') {
      $children = get_terms('category', array('child_of' => $item->object_id));

      if (!empty($children) && !is_wp_error($children)) {

        $output .= '<ul class="sub-menu">';
        if (isset($children) && !is_wp_error($children) && sizeof($children) > 0) {
          $child_items = '';
          foreach ($children as $child) {
            $child_term = get_term($child, 'category');
            $child_item     =   '<li id="term-item-' . $child_term->term_id . '" class="menu-item menu-item-type-taxonomy menu-item-object-category term-item-' . $child_term->term_id . '"><div class="ancestor-wrapper">';
            $child_item     .=  '<a href="' . get_term_link($child_term, 'category') . '">' . $child_term->name . '</a>';
            $child_item     .=  '</div></li>';
            $child_items    .=  $child_item;
          }
          $output         .= $child_items;
        }
        $output .= '</ul>';
      }
    }

    $output .= "</li>\n";
  }
}

Add this to your functions.php and then call the walker like this:

<nav>
	<?php
	wp_nav_menu(array(
		'theme_location' => 'main_menu',
		'menu_id'        => 'main_menu',
		'container' => false,
		'walker' => new Walker_Add_Myterms(),
	));
	?>
</nav>