Fatal error: Uncaught Error: Class "Drupal\forum\Plugin\Block\ForumBlockBase" not found in /home/www/drupal8/test/core/modules/forum/src/Plugin/Block/ActiveTopicsBlock.php:14 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/forum/src/Plugin/Block/ActiveTopicsBlock.php on line 14