Fatal error: Uncaught Error: Class "Drupal\Core\Block\BlockBase" not found in /home/www/drupal8/test/core/modules/system/src/Plugin/Block/SystemMainBlock.php:19 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/system/src/Plugin/Block/SystemMainBlock.php on line 19