WordPress: set category menu active on post single page

function.php

<?php

/**
 * helper function to test if post is in subcategory
 */
if (!function_exists("mwe_is_in_subcategory")) {
    function mwe_is_in_subcategory($categories, $_post = null)
    {
        foreach ((array) $categories as $category) {
            $subcats = get_term_children((int) $category, "category");
            if ($subcats && in_category($subcats, $_post)) {
                return true;
            }
        }
        return false;
    }
}

/**
 * Set menu active css classes of categories
 * Example for CPT "produkt"
 */
add_action("nav_menu_css_class", "add_current_nav_class", 10, 2);

function add_current_nav_class($classes, $item)
{
    // Getting the current post details
    $post = get_queried_object();

    if ($item->object == "category" && $post->post_type == "produkt") {
        // is the post in a subcategory of this category?
        if (mwe_is_in_subcategory($item->object_id)) {
            $classes[] = "current-menu-parent";
        }

        // is post in this category?
        if (in_category($item->object_id, $post)) {
            $classes[] = "current-menu-item";
        }
    }

    // Return the corrected set of classes to be added to the menu item
    return array_unique($classes);
}