Fix learndash_woocommerce.php + WooCommerce Subscription rollback issue

fix-woo-learndash-sub-rollback.php

<?php
/**
 * Theme filters.
 */
namespace App;

/**
 * Fix checkout bug with learndash_woocommerce
 * 
 * learndash_woocommerce.php:580
 * learndash_woocommerce.php:606
 * 
 * Those lines use $_POST['customer_user'] regardless of admin or frontend and cause a fatal error
 * since the key doesn't exist (or doesn't always exist) on frontend checkout process. WooCommerce Subscription
 * then rollbacks its transaction that saves the subscription line line items to the subscription, hence not
 * being able to see the amounts, renew manually, deactivate renewals or plainly being unable to renew orders.
 */
function make_sure_post_key_exists_to_a_null_value($order_id) {
    if(!isset($_POST['customer_user'])) {
        $order = wc_get_order( $order_id );
        $_POST['customer_user'] = $order->get_customer_id();
    }
}
add_action('woocommerce_process_shop_order_meta', __NAMESPACE__ . '\\make_sure_post_key_exists_to_a_null_value', 0, 1);
add_action('woocommerce_before_subscription_object_save', __NAMESPACE__ . '\\make_sure_post_key_exists_to_a_null_value', 0, 1);