Fatal error: Uncaught Error: Class "Drupal\Core\Plugin\PluginFormBase" not found in /home/www/drupal8/test/core/modules/system/src/Form/SystemMenuOffCanvasForm.php:24 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/system/src/Form/SystemMenuOffCanvasForm.php on line 24