8 – Upgrade Status module reports PHPUnitFrameworkTestCase is not an interface

Our site is running Drupal 8.8.13 and we want to move to Drupal 9. I’ve installed the Upgrade Status module. When I scan a module, like Admin Toolbar, it reports the problem:

web/modules/contrib/admin_toolbar/admin_toolbar_search/admin_toolbar_search.module  

Internal error: PHPUnitFrameworkTestCase is not an interface 
Run PHPStan with --debug option and post the stack trace to: 
https://github.com/phpstan/phpstan/issues/new?template=Bug_report.md

Lots of other modules have the same problem.

Is this warning legit? or is there a problem with the configuration of Upgrade Status module?