code: private function getSessionBag(Request $request): SessionBagInterface
      {
          if (!$request->hasSession() || null === ($session = $request->getSession())) {
              throw new \RuntimeException('The request did not contain a session.');
          }
  
          $name = 'contao_frontend';
  
          if ($this->scopeMatcher->isBackendRequest($request)) {
              $name = 'contao_backend';
          }
  
          return $session->getBag($name);
      }
docs: Returns the session bag.
  
  @throws \RuntimeException