[Solved] Upgrade Status module reports PHPUnitFrameworkTestCase is not an interface

bwaindwain Asks: 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 v3.3. When I scan a module, like Admin Toolbar, it reports the problem:

Code:
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?

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