Refreshing a page of not existing group cause 2 errors

Describe the bug

Problem has been detected in this PR: https://github.com/nextcloud/contacts/pull/2744#issuecomment-1131509827

Steps to reproduce

1. Create a new group
2. Add one user to this group
3. Go to /apps/contacts/nameOfGroup
4. In a contact view: remove this group from the checkmark within the group dropdown
5. Refresh a page (F5)

Expected behavior

No errors

Actual behavior

User see 2 error messages:

• Group group not found
• Contact not found

Contact version

4.2.0

Operating system

not relevant

PHP engine version

No response

Web server

No response

Database

No response

Operating system

not relevant

List of activated Apps

Contacts

Nextcloud Signing status

No response

Configuration report

No response

Additional info

CardDAV groups are similar to "tags". They could only take place, if there is at least one contact. If last contact goes away from a group, the whole group will disappear. We could not change this behavior.

After page reload the contact don't know, which group does't exist anymore.

To fix this bug: After deleting a last contact from a group -> navigate somewhere else (for example to this contact)

Start by reproducing the group creation, last-contact removal, navigation to /apps/contacts/nameOfGroup, and page refresh described in the issue. Trace the group and contact state used after reload; done means the stale group page no longer shows either “Group not found” or “Contact not found” errors.