Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\PluginBase" not found in /home/www/drupal8/test/core/lib/Drupal/Core/Layout/LayoutDefault.php:11 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/lib/Drupal/Core/Layout/LayoutDefault.php on line 11