Fatal error: Uncaught Error: Class "Symfony\Component\Validator\ConstraintViolationList" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Entity/EntityConstraintViolationList.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Entity/EntityConstraintViolationList.php on line 15