code: public function onDelete(ResourceEventInterface $event)
      {
          $item = $this->getSupplierOrderItemFromEvent($event);
  
          $this->assertDeletable($item);
  
          // TODO If not made by the supplierOrderListener ?
          //$this->deleteSupplierOrderItemStockUnit($item);
          $this->stockUnitLinker->unlinkItem($item);
  
          // Supplier order has been set to null by the removeItem method.
          // Retrieve it from the change set.
          if (null === $order = $item->getOrder()) {
              $changeSet = $this->persistenceHelper->getChangeSet($item);
              if (array_key_exists('order', $changeSet)) {
                  $order = $changeSet['order'][0];
              }
          }
          if (null === $order) {
              throw new RuntimeException("Failed to retrieve supplier order.");
          }
  
          // Clear association
          $item->setOrder(null);
          /* @see SupplierDeliveryListener::onDelete */
          //$order->getItems()->removeElement($item);
  
          // Trigger the supplier order update
          if (!$this->persistenceHelper->isScheduledForRemove($order)) {
              $this->scheduleSupplierOrderContentChangeEvent($order);
          }
      }
docs: Delete event handler.
  
  @param ResourceEventInterface $event