Layout Builder "manage layout" breaks when bundle is not an entity type

Problem/Motivation

I am working on Layout Builder integration with CiviCRM Entity. We have bundles, but they are not config entities. They are defined via the bundles hook.

When enabling Layout Builder and accessing the manage layout form, we receive a 404. I believe this is the offending code:

      // If the entity type has no bundles and it doesn't use {bundle} in its
      // admin path, use the entity type.
      if (strpos($path, '{bundle}') === FALSE) {
        if (!$entity_type->hasKey('bundle')) {
          $defaults['bundle'] = $entity_type_id;
        }
        else {
          $defaults['bundle_key'] = $entity_type->getBundleEntityType();
        }
      }

The bundle_key comes from getBundleEntityType not the bundle key itself. The current code causes the defaults for the route to be null since it's not a bundle entity type.

Steps to reproduce

Enable Layout Builder for an entity that has non-entity bundles.

Proposed resolution

Get the key from the entity type.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet