Fixed recently introduced notifications related null reference exceptions

[Mailaender]

Closes #19837

[pchote]

I don't see how these changes could avoid any NREs inside PlayPredefined. What am I missing?

[Mailaender]

I believe the crash to be related to self.World.Map.Rules where self is disposed.

[pchote]

INotifyKilled.Killed is never called on a disposed actor (it is called as the last thing before disposing) - if this is somehow happening, then it needs to be fixed at the source!

[Mailaender]

It is clearly a regression from #19667 that I don't yet understand.

[Mailaender]

I found a fix. Not sure why the stacktraces are so misleading.

[RoosterDragon]

Is the issue here that a null sound ends up in currentNotifications which later hurts us? In that case, would it make more sense to add a check after var sound = soundEngine.Play2D(... (line 410) and do if (sound == null) return false;? That would tell the caller the sound failed to play, and stops us putting nulls into either currentNotifications or currentSounds which makes life easier to think about.

[Mailaender]

Added that as well. I don't know why the notifications are null. The rules seem to be correct.

[Dzierzan]

I am sorry to be an ass, but can someone review that? We wanted to release another pre-release version of OpenHV and we need this fix.

[abcdefg30]

Untested but changes lgtm.

[reaperrr]

https://github.com/OpenRA/OpenRA/wiki/Changelog-(bleed)/_compare/8e9317262f3ec889028be03a11bf16b931977a53...d40247b1d27a8886766c6e9708741aba641110d1