I'm not sure what's causing this. I'm using Armored Train (Turret Wagons) by pandamiami80s. This is happening while biters are clumping together to begin an attack on our walls. We have a flamethrower turret, and 2 gun turrets adjacent to the spotlight. They appear to function as intended for a few minutes, then this crash will occur.

The mod ⚠️ SearchlightAssault ⚠️ (2.3.19) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SearchlightAssault::on_script_trigger_effect (ID 173)
SearchlightAssault/control-gestalt.lua:192: attempt to index local 'tu' (a nil value)
stack traceback:
SearchlightAssault/control-gestalt.lua:192: in function 'BoostFriends'
SearchlightAssault/control-gestalt.lua:224: in function 'EnterAlarmMode'
SearchlightAssault/control-gestalt.lua:517: in function 'FoeFound'
SearchlightAssault/control.lua:398: in function <SearchlightAssault/control.lua:391>

I'm having trouble recreating this, any chance you send me your save file?

I'd think it's weird, because it looks like same problem someone else reported on this forum a year or so ago that was fixed. Here's the google drive share link:
https://drive.google.com/file/d/1j15G2pQAc7U_eO3xkdGa5VGY4yiG7SAA/view?usp=drive_link

Thank you for sending your save, it was really helpful!
I've just released version 2.3.20 which should fix this issue.
The new version should start to appear in the mod portal shortly.
If you don't see it in-game you can download it manually here: https://mods.factorio.com/mod/SearchlightAssault/downloads

Please let me know if you see further issues!