public function registerSubscriptions(EventDispatcher $dispatcher)
    {
        if ($this->registered) {
            throw new \RuntimeException('Cannot register debug toolbar subscriptions when already registered.');
        }

        foreach ($this->toolbar->getExtensions() as $extension) {
            if ( ! $extension instanceof EventSubscriberInterface) {
                continue;
            }

            $dispatcher->addSubscriber($extension);
        }

        $this->registered = true;
    }