Fatal error: Uncaught Error: Class "Drupal\Core\Field\WidgetBase" not found in /home/www/drupal8/test/core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php:36 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php on line 36