Fatal error: Uncaught Error: Class "Symfony\Component\HttpKernel\Exception\PreconditionRequiredHttpException" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Http/Exception/CacheablePreconditionRequiredHttpException.php:12 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Http/Exception/CacheablePreconditionRequiredHttpException.php on line 12