Fatal error after upgrading to 3.0.7: Typed property $cacheableUrlMetadata accessed before initialization

Problem/Motivation

After upgrading rest_menu_items from 3.0.6 to 3.0.7, requests to the REST menu endpoint fail with a PHP fatal error.

Error:

Error: Typed property
Drupal\rest_menu_items\Plugin\rest\resource\RestMenuItemsResource::$cacheableUrlMetadata
must not be accessed before initialization in
Drupal\rest_menu_items\Plugin\rest\resource\RestMenuItemsResource->captureGeneratedUrl()
(line 516 of modules/contrib/rest_menu_items/src/Plugin/rest/resource/RestMenuItemsResource.php).

The issue did not occur in version 3.0.6 and started immediately after updating to 3.0.7.

Steps to reproduce

1. Install rest_menu_items 3.0.6.
2. Configure and verify that the REST menu endpoint is working.
3. Update the module to version 3.0.7.
4. Clear caches.
5. Request a menu endpoint.
6. Observe the fatal error.

Proposed resolution

Investigate initialization of the cacheableUrlMetadata property. It appears that captureGeneratedUrl() can access the property before it has been initialized.

Ensure cacheableUrlMetadata is instantiated before use, or add a null/initialization check before accessing it.

Remaining tasks

- Reproduce issue on a clean installation.
- Identify the code path that skips initialization.
- Create and test a patch.
- Add automated test coverage.

User interface changes

None.

API changes

None expected