Fatal error: Uncaught Error: Interface "Drupal\Core\Ajax\CommandInterface" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Ajax/AddCssCommand.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Ajax/AddCssCommand.php on line 15