Fatal error: Uncaught Error: Class "Symfony\Component\Validator\ConstraintValidator" not found in /home/www/drupal8/test/core/modules/book/src/Plugin/Validation/Constraint/BookOutlineConstraintValidator.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/book/src/Plugin/Validation/Constraint/BookOutlineConstraintValidator.php on line 14