Error on setting conference invisible.

[snicki13]

Describe the bug
Conference is immediately displayed as empty and removed from the list, as it is set invisible.
Error: Conference 0641b12b-778d-4ed6-93b6-f790ab28189b not found in attendedConferences!

To Reproduce
Race condition. See logs.

Expected behavior
No error.

Additional context
2021-11-16 22:20:21.191 INFO 1 --- [or-http-epoll-3] d.t.m.i.c.services.ConferenceService : Dominik Kröll joins conference 0641b12b-778d-4ed6-93b6-f790ab28189b!

2021-11-16 22:20:22.819 INFO 1 --- [ scheduling-1] d.t.m.i.c.services.ConferenceService : Updating conferences of Datenbanksysteme - Übungsraum.

2021-11-16 22:20:22.869 DEBUG 1 --- [or-http-epoll-3] d.t.m.i.c.services.ConferenceService : Conference 0641b12b-778d-4ed6-93b6-f790ab28189b scheduled for deletion if still empty in 30 seconds!

2021-11-16 22:20:52.869 DEBUG 1 --- [ parallel-2] d.t.m.i.c.services.ConferenceService : Conference 0641b12b-778d-4ed6-93b6-f790ab28189b is still empty. Deleting...