我一直在使用此代码来处理续期付款。问题是这条线

 $parent_id                  = WC_Subscriptions_Renewal_Order::get_parent_order_id( $subscription ); // <<<<< this is failing

最终会导致致命错误：

 Fatal error: Uncaught Error: Call to a member function get_parent_id() on null in ....public_html/wp-content/plugins/woocommerce-subscriptions/includes/class-wc-subscriptions-renewal-order.php:315

目前我的代码是这样：

 add_action( 'woocommerce_subscription_renewal_payment_complete','mmd_woocommerce_subscription_renewal_payment_complete',1,2);      // The subscription renewal payment
 function mmd_woocommerce_subscription_renewal_payment_complete($subscription,$last_order)
 {
 global $wpdb;

 $SubscriptionNumber         = $subscription->get_order_number();
 $BusinessEmail              = $subscription->get_billing_email();

 $OrderNumber                = $subscription->parent_id;

 //***********************************
 // Another way to get the Parent Order from the subscription.
 $parent_id                  = WC_Subscriptions_Renewal_Order::get_parent_order_id( $subscription ); // <<<<< this is failing
 $ParentOrder                = new WC_Order( $parent_id );
//***********************************


 foreach ( $subscription->get_items() as $item ) 
   { 
   $NewExpiration = WC_Subscriptions_Order::get_next_payment_date ( $ParentOrder,$item['product_id'] ); /* This should get the next payment date... */
   if ( $NewExpiration )
      {
      Do some work...
      }
  }
 }

我需要另一种获取父订单ID的方法。有什么想法吗？