Invoked by the terminate kernel event.
\Symfony\Component\HttpKernel\Event\PostResponseEvent $event: The event object.
public function onKernelTerminate(PostResponseEvent $event) {
  foreach ($this->services as $id) {
    // Check if the service was initialized during this request, destruction
    // is not necessary if the service was not used.
    if ($this->container
      ->initialized($id)) {
      $service = $this->container
        ->get($id);
      $service
        ->destruct();
    }
  }
}