Symfony 3.4: Services are private by default
Symfony 3.4: Services are private by default https://symfony.com/blog/new-in-symfony-3-4-services-are-private-by-default
Symfony\Component\DependencyInjection\Compiler\ResolvePrivatesPass removes services marked as private from the container even if they are also marked as public.
The definition for the action-provider service has the setting
private: true. Therefore the service is not available if the CMS depends on Symfony ^3.4 or ^4.0. This affects Drupal 9 and probably ^8.8 (not tested).
setPrivate(FALSE) solves this issue.