There was a bug report 5 months ago talking about this exact same problem but it's happening again, I'll give a bit more information this time though:
Error while running event robotarmy::on_tick (ID 0)
robotarmy/robolib/Squad.lua:658: attempt to perform arithmetic on field '?' (a nil value)

Edit: Sorry for putting this in general, didn't realize my mistake until after I posted and I don't know how to fix it.

Just got exactly the same error :
Error while running event robotarmy::on_tick (ID 0)
robotarmy/robolib/Squad.lua:658: attempt to perform arithmetic on field '?' (a nil value)
Edit : I'm playing with "Brave New World" mod (only mod used). The bug crashes the game, I can't play my game anymore.

Hi guys,

Sorry to hear you are having issues with this. It's been reported numerous times on my Git issues as well.

i'll get a patch out tonight at least to fix that bug, my other long-term feature additions/patches can wait.

Tks for your answer. I continued without the mod, but I got so used to it that I can barely play without it now (on the Brave New World mod, it's a "must have"), I'll really enjoy if it's up again :-)
Tks for your hard work, and can't wait for what you call a "long-term feature" :-)

Okay, update to 0.3.7 and see if that fixes things for you.