BrowserTestBase::$defaultTheme is required

1) Drupal\Tests\tour_ui\Functional\TourUITest::testUi
Exception: Drupal\Tests\BrowserTestBase::$defaultTheme is required. See https://www.drupal.org/node/3083055, which includes recommendations on which theme to use.

https://www.drupal.org/node/3083055 recommends not to
protected $defaultTheme = 'stark';