After upgrading to CiviCRM 5.27 (and now, to 5.28) on Drupal 7, I see this warning message.
As you can see, GROUP is listed as "-" (minus).
When I try to click on settings (Gear icon), it shows me Group Settings empty fields:
When I click on Contacts in this Group (in Group Settings), it shows me all contacts in my CiviCRM database along with some notice warning (a lot of lines exactly like these - Undefined property and Undefined Index):
(January, 2021) The problem is still there after update to 5.33.2. What can I do about that?