Fatal error: Uncaught Error: Class "Drupal\Core\Config\Schema\ArrayElement" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Config/Schema/Sequence.php:20 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Config/Schema/Sequence.php on line 20