Since request #44449, the "News" service and everything linked to it has been removed. But the REST route GET /project/{id}/user_groups
with "system" user groups would find the "news admin" and "news writers" system user groups. It raised a PHP notice because it tried to find their IDs in an array.
It should not raise a notice, and should not try to find the "news admin" and "news writers" user groups, even if they are still in the database. It should skip them.