Compatibility issue with WooCommerce 5.0.0 and Compatibility mode

Hi,

When enabling the 404page compatibility mode on a clean WooCommerce installation and an order programatically goes from “Pending” to “Processing” (read: some piece of code uses WC_Order->payment_complete()) then the fatal error shown below appears.

So; how to test:

1. Create fresh installation and install 404page and WooCommerce (latest 5.0.0)
2. Create a new order and set it to “Pending” in the UI
3. Either find a payment gateway that will run WC_Order->payment_complete() or use the code snippet below

Why is it 404page?

The short answer is that the error stops occurring when 404page is disabled and I’ve specifically narrowed it down to add_filter( 'the_posts', array( $this, 'show404_compatiblity_mode' ), 999 ); in the class-404page.php file. Removing this filter will make the WooCommerce email during payment complete retrieve posts.

How to reproduce example

add_action( 'init', function () {
	// add "?test=test" to the URL to avoid this being run on all requests. Or just remove it completely. It's up to you!
	if ( ! isset( $_GET['test'] ) ) {
		return;
	}

	// Change 123 to your orders ID.
	$order = wc_get_order(123);
	$order->payment_complete();
} );

Error message

#0 /var/www/html/wp-content/plugins/woocommerce/templates/emails/email-order-items.php(75): WC_Order->get_qty_refunded_for_item(244)
#1 /var/www/html/wp-content/plugins/woocommerce/includes/wc-core-functions.php(345): include('/var/www/html/w...')
#2 /var/www/html/wp-content/plugins/woocommerce/includes/wc-template-functions.php(3247): wc_get_template('emails/email-or...', Array)
#3 /var/www/html/wp-content/plugins/woocommerce/templates/emails/email-order-details.php(56): wc_get_email_order_items(Object(Automattic\WooCommerce\Admin\Overrides\Order), Array)
#4 /var/www/html/wp-content/plugins/woocommerce/includes/wc-core-functions.php(345): include('/var/www/html/w...')
#5 /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-emails.php(421): wc_get_template('emails/email-or...', Array)
#6 /var/www/html/wp-includes/class-wp-hook.php(287): WC_Emails->order_details(Object(Automattic\WooCommerce\Admin\Overrides\Order), false, false, Object(WC_Email_Customer_Processing_Order))
#7 /var/www/html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#8 /var/www/html/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#9 /var/www/html/wp-content/plugins/woocommerce/templates/emails/customer-processing-order.php(40): do_action('woocommerce_ema...', Object(Automattic\WooCommerce\Admin\Overrides\Order), false, false, Object(WC_Email_Customer_Processing_Order))
#10 /var/www/html/wp-content/plugins/woocommerce/includes/wc-core-functions.php(345): include('/var/www/html/w...')
#11 /var/www/html/wp-content/plugins/woocommerce/includes/wc-core-functions.php(364): wc_get_template('emails/customer...', Array, '', '')
#12 /var/www/html/wp-content/plugins/woocommerce/includes/emails/class-wc-email-customer-processing-order.php(113): wc_get_template_html('emails/customer...', Array)
#13 /var/www/html/wp-content/plugins/woocommerce/includes/emails/class-wc-email.php(541): WC_Email_Customer_Processing_Order->get_content_html()
#14 /var/www/html/wp-content/plugins/woocommerce/includes/emails/class-wc-email-customer-processing-order.php(93): WC_Email->get_content()
#15 /var/www/html/wp-includes/class-wp-hook.php(287): WC_Email_Customer_Processing_Order->trigger(5335, Object(Automattic\WooCommerce\Admin\Overrides\Order))
#16 /var/www/html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#17 /var/www/html/wp-includes/plugin.php(551): WP_Hook->do_action(Array)
#18 /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-emails.php(170): do_action_ref_array('woocommerce_ord...', Array)
#19 /var/www/html/wp-includes/class-wp-hook.php(287): WC_Emails::send_transactional_email(Array, Object(Automattic\WooCommerce\Admin\Overrides\Order))
#20 /var/www/html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters('', Array)
#21 /var/www/html/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#22 /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-order.php(372): do_action('woocommerce_ord...', 5335, Object(Automattic\WooCommerce\Admin\Overrides\Order))
#23 /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-order.php(221): WC_Order->status_transition()
#24 /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-order.php(119): WC_Order->save()
#25 /var/www/html/wp-content/plugins/some-gateway/includes/class-some-wc-gateway-invoice.php(181): WC_Order->payment_complete()
#26 /var/www/html/wp-includes/class-wp-hook.php(287): Some_WC_Gateway_Invoice->callback_handler('')
#27 /var/www/html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters('', Array)
#28 /var/www/html/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#29 /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-api.php(161): do_action('woocommerce_api...')
#30 /var/www/html/wp-includes/class-wp-hook.php(287): WC_API->handle_api_requests(Object(WP))
#31 /var/www/html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#32 /var/www/html/wp-includes/plugin.php(551): WP_Hook->do_action(Array)
#33 /var/www/html/wp-includes/class-wp.php(388): do_action_ref_array('parse_request', Array)
#34 /var/www/html/wp-includes/class-wp.php(745): WP->parse_request('')
#35 /var/www/html/wp-includes/functions.php(1291): WP->main('')
#36 /var/www/html/wp-blog-header.php(16): wp()
#37 /var/www/html/index.php(17): require('/var/www/html/w...')
#38 {main}
  thrown in /var/www/html/wp-content/plugins/woocommerce/includes/class-wc-order.php on line 1938