code: public function process(ContainerBuilder $container): void
      {
          if (!$container->hasDefinition('session')) {
              return;
          }
  
          $session = $container->findDefinition('session');
          $session->addMethodCall('registerBag', [new Reference('contao.session.contao_backend')]);
          $session->addMethodCall('registerBag', [new Reference('contao.session.contao_frontend')]);
      }
docs: {@inheritdoc}