Extending search in backend orders list for product items by id or by sku

Im trying to use the following code from Search by order item SKU or ID in Woocommerce Orders Admin page to enable searching woocommerce orders by sky and id, sku being the important part for me.

   add_filter( 'woocommerce_shop_order_search_fields', function ($search_fields ) {
    $posts = get_posts(array('post_type' => 'shop_order'));

    foreach ($posts as $post) {
        $order_id = $post->ID;
        $order = new WC_Order($order_id);
        $items = $order->get_items();

        foreach($items as $item) {
            $product_id = $item['product_id'];
            $search_sku = get_post_meta($product_id, "_sku", true);
            add_post_meta($order_id, "_product_sku", $sku);
            add_post_meta($order_id, "_product_id", $product_id);
        }
    }

    return array_merge($search_fields, array('_product_sku', '_product_id'));
});

When I add this to my functions.php I get the following errors:

Array() expects parameter 1 to be a valid callback, function 'woocommerce_shop_order_search_order_total' not found or invalid function name in /var/sites/s/http://ift.tt/1UImqBq on line 235

Warning: array_merge(): Argument #1 is not an array in /var/sites/s/http://ift.tt/1UIlflM on line 156

Warning: array_map(): Argument #2 should be an array in /var/sites/s/http://ift.tt/1UImiSt on line 1533

Warning: array_map(): Argument #2 should be an array in /var/sites/s/http://ift.tt/1UImiSt on line 1557

Warning: implode(): Invalid arguments passed in /var/sites/s/http://ift.tt/1UImiSt on line 1557

Im making the assumption that since this is about 1 year old and woocommerce have gone through some big changes in that time, this code is needs updating in someway however i am not experienced enough with woocommerce to recognise what is wrong.

If anyonce out there is able to just confirm I am on the correct path or offer guidence / suggestions as to what I might need to do that would be great.

Thanks

via Chebli Mohamed