ThemeEngine BC layer is broken - ServiceLocator != Container

Problem/Motivation

📌 Convert theme engines into services Needs work needs a quick fix because a service locator does not support $this->themeEngines->get($name, SymfonyContainerInterface::NULL_ON_INVALID_REFERENCE);

Steps to reproduce

See 🐛 Semi-coupled engine not compatible with Drupal 11.x Active

Proposed resolution

Use has()

Remaining tasks

User interface changes

None

Introduced terminology

N/a

API changes

None

Data model changes

None

Release notes snippet

N/a