I use this mod and the mod Coin generator and I've experienced this error when killing 2 enemies at the same moment with different weapons, like personal defense and a weapon or a weapon and an effect.

The mod RPG System (0.18.43) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event RPGsystem::on_entity_damaged (ID 97)
The mod Coin Generation (18.0.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event coin_generation::on_entity_died (ID 4)
coin_generation/scripts/biter-killing.lua:92: attempt to index local 'killer' (a nil value)
stack traceback:
coin_generation/scripts/biter-killing.lua:92: in function 'handlerFunction'
coin_generation/utility/events.lua:104: in function '_HandleFilteredEvent'
coin_generation/utility/events.lua:21: in function <coin_generation/utility/events.lua:19>
stack traceback:
RPGsystem/control.lua:1279: in function <RPGsystem/control.lua:1227>
stack traceback:
[C]: in function 'damage'
RPGsystem/control.lua:1279: in function <RPGsystem/control.lua:1227>

It seems happen even if you use an area weapon like granade or rockets...
Just tried and got the same error.

Looks like an error on the other mod

I've reported it to both modders.
I'm not exactly sure about what create it, but after deactivating your I've received this:

The mod Coin Generation (18.0.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event coin_generation::on_entity_died (ID 4)
coin_generation/scripts/biter-killing.lua:92: attempt to index local 'killer' (a nil value)
stack traceback:
coin_generation/scripts/biter-killing.lua:92: in function 'handlerFunction'
coin_generation/utility/events.lua:104: in function '_HandleFilteredEvent'
coin_generation/utility/events.lua:21: in function <coin_generation/utility/events.lua:19>

Then I dont think is your mod too...
Sorry, but reporting it is the best solution, so both of you can try to fix the interaction problem, or the main problem, if it's only on one mod...
Thanks for your fast answer, anyway. Not all modders are so fast.

It has been fixed. If someone is worring about it, there's no more problems.