Fatal error: Uncaught Error: Interface "Drupal\Core\Theme\ThemeNegotiatorInterface" not found in /home/www/drupal8/test/core/modules/user/src/Theme/AdminNegotiator.php:15 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/user/src/Theme/AdminNegotiator.php on line 15