TOPs sometime have no name

https://sentry.fsmpi.rwth-aachen.de/protokollsystem/protokollsystem/issues/17/

TypeError: sequence item 8: expected str instance, NoneType found
(3 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())

TypeError: sequence item 8: expected str instance, NoneType found