code: public function onPreDelete(ResourceEventInterface $event)
      {
          $delivery = $this->getSupplierDeliveryFromEvent($event);
  
          $this->assertDeletable($delivery);
  
          // Initialize the supplier order's deliveries collection before the delivery removal.
          // @see http://stackoverflow.com/questions/41102378/scheduled-entity-in-onflush-is-different-instance#answer-41361138
          $delivery->getOrder()->getDeliveries()->count();
      }
docs: Pre delete event handler.
  
  @param ResourceEventInterface $event
  
  @throws Exception\IllegalOperationException