Fatal error: Uncaught Error: Interface "Drupal\Core\Theme\ThemeNegotiatorInterface" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Theme/DefaultNegotiator.php:11 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Theme/DefaultNegotiator.php on line 11