Please Use Below code
<?php
$addcats = array();
ob_start();
require 'custom_menu_addon.php';
$addCats = ob_get_clean();
wp_nav_menu(array(
'theme_location' => 'menu-1',
'menu_id' => 'primary-menu',
'items_wrap' => '<ul id="%1$s" class="%2$s">%3$s<div class="my-div">' . $addCats . '</div></ul>'
));
I’ve edited this answer after some research into the usage of this object array and put it in context with my code, works well on the site 🙂