Auto flag after user registers does not work for me.

I was getting below error after login/register
Fatal error: Declaration of Drupal\flag_anon\FlagAnonLinkBuilder::build($entity_type_id, $entity_id, $flag_id) must be compatible with Drupal\flag\FlagLinkBuilder::build($entity_type_id, $entity_id, $flag_id, $view_mode = null) in /app/web/modules/contrib/flag_anon/src/FlagAnonLinkBuilder.php on line 73
I have raised MR for the same,and now it works.
Please review

I was able to reproduce the issue and encountered the same error. However, it was resolved after applying MR!9. I'm attaching a screenshot of the error. Moving the issue to RTBC.