When I kill more than one critter using both personal defense lasers and hand weapons, I receive this message. Tried twice and it's happened again, then I'm reporting it.
After the message, the game stop and you're sent to home page.

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.

Deactivating RPG system I've got this message in the same conditions:

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>

thanks for the report.
update published that resolves the issue when it occurs without RPG mod present.
I tested on the fixed version with the RPG mod active and saw no other errors. So it looks to be a false/misleading inclusion of the RPG mod in the error report.
Side note, the error occured when Factorio API had no killer listed for the enemies death. In this case the coins will drop on the ground as a fallback, as the mod doesn't know who caused the enemie's death.

I've reported as it seems present, but having even coin generation codes inside I've reported it to you too.
I've then tried it wothout RPG mod and it has been up again, and I've updated the report.
Thanks for the fix.
I really appreciate your quick fix, but just call you've seen the report.
I was worried there was no hope to solve the problem soon because you've not given a sign of reading the report. :P
My bad habitudes, sorry.

tbh from when I looked at it properly to having it reproduced, fixed and published was like an hour.

Wow! A true turbofix! :D :D :D