Fatal error: Uncaught Error: Interface "Drupal\Core\Block\BlockPluginInterface" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Block/TitleBlockPluginInterface.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Block/TitleBlockPluginInterface.php on line 14