Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\PluginBase" not found in /home/www/drupal8/test/core/modules/help/src/Plugin/HelpSection/HelpSectionPluginBase.php:16 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/help/src/Plugin/HelpSection/HelpSectionPluginBase.php on line 16