Tobias Bähr
--- a/core/lib/Drupal/Core/Menu/LocalTaskManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
 use Drupal\Core\Routing\RouteProviderInterface;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Url;
+use Psr\Log\LoggerInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
 use Symfony\Component\HttpKernel\Controller\ArgumentResolverInterface;
+use function trigger_error;
--- a/core/lib/Drupal/Core/Menu/LocalTaskManager.php
+++ b/core/lib/Drupal/Core/Menu/LocalTaskManager.php
            // reference like &$task_info causes bugs.
            $definitions[$plugin_id]['base_route'] = $definitions[$task_info['parent_id']]['base_route'];
          }
+          if (!isset($task_info['route_name'])) {