Fatal error: Uncaught Error: Class "Drupal\Core\Config\Schema\ArrayElement" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Config/Schema/Mapping.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Config/Schema/Mapping.php on line 18