Fatal error: Uncaught Error: Class "Symfony\Component\EventDispatcher\Event" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Render/PageDisplayVariantSelectionEvent.php:22 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Render/PageDisplayVariantSelectionEvent.php on line 22