Do not fail top-listing when localtops are missing
https://sentry.fsmpi.rwth-aachen.de/protokollsystem/protokollsystem/issues/16/
AttributeError: 'NoneType' object has no attribute 'description'
(5 additional frame(s) were not displayed)
...
File "program/server.py", line 1862, in feed_all_appointments_ical
return make_calendar_from_protocols(protocols, "Sitzungskalender")
File "program/server.py", line 1827, in make_calendar_from_protocols
top.name for top in protocol.get_tops())
AttributeError: 'NoneType' object has no attribute 'description'