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