Fatal error: Uncaught Error: Class "Drupal\Core\Template\AttributeValueBase" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Template/AttributeString.php:22 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Template/AttributeString.php on line 22