This was always haphazard code, but by luck it never triggered before
drawable load until now. With the recently nullability changes, this
would be triggered when `flash` is not yet constructed.
Switching to `AddOnce` seems safer to avoid multiple applications,
regardless.
Was causing there to be two kinds of missing `TournamentBeatmap`s: one
with missing `Beatmap == null`, and one with `Beatmap == new
APIBeatmap()`. In particular, they would appear differently in UI
(either as "unknown - unknown", or the intensely enigmatic " - ").
Because nobody else should need to do anything as cursed as what
`ScheduleScreen` was doing (which is 95% of the reason why the selection
box even broke there).