code: private function initializeLegacySessionAccess(): void
      {
          if (!$session = $this->getSession()) {
              return;
          }
  
          if (!$session->isStarted()) {
              $_SESSION = new LazySessionAccess($session);
          } else {
              $_SESSION['BE_DATA'] = $session->getBag('contao_backend');
              $_SESSION['FE_DATA'] = $session->getBag('contao_frontend');
          }
      }
docs: Initializes session access for $_SESSION['FE_DATA'] and $_SESSION['BE_DATA'].