this is a stupid thing to do but i managed to do it

steps to reproduce:
get a few copper coins
open the wallet gui
attempt to put the coins in the wallet slot while it's empty

crash log: https://pastebin.com/240j5R1J

What version of Lightman's Currency were you using, cause I supposedly fixed this issue a while ago... It shouldn't be possible to put anything that's not a wallet in the wallet slot on any version 1.0.1.6 or newer.

That said, it also shouldn't be possible to only have 22 slots in the wallet screen (minimum should by 37, 36 player inventory slots + the wallet slot), so my guess is the screen tick and the click processing happened in parallel and the slots were refreshed while the click processing was in the middle of a for loop checking the menu slots...

It's really all down to luck sadly, which makes it a b**** to test to see if my "fixes" for these issues are actually doing anything unfortunately. That said, the issue with being able to put coins in there at all should have been long since fixed, so I stand by what I said/requested in my previous comment.

Since you have not responded informing me of which Lightman's Currency version you're using, and I am unable to replicate the issue, I going to assume this is the same or a similar issue to issue #13 that I've already since corrected.
In v1.0.2.3, for 1.18 I also added some additional internal changes to the wallet screen that should prevent any and all issues with slot counts mis-matching as well, which should permanently fix or at least significantly decrease the chances of encountering a crash such as this ever again.