Fatal error in PHP 8.1

The fatal error is displayed when the status of an order is updated to “Completed” in WooCommerce in the WordPress administration. Just open an order in the administration, change the order status to “Completed” in the dropdown and click the “Update” button.

It can be fixed by changing the method on line 66 in “wp-content/plugins/newsletter-optin-box/includes/automation-rules/triggers/class-noptin-woocommerce-product-purchased-trigger.php” (maybe_trigger) from protected to public.