Fatal error: Uncaught Error: Interface "Drupal\Core\Extension\ThemeInstallerInterface" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Extension/ThemeInstaller.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Extension/ThemeInstaller.php on line 18