This is a bit of an odd one, as I only got it to happen once. Any attempts at reproducing the crash have failed so far, and from my understanding two conditions need to be true simultaneously:
- The upgrade must result in a different number of modules slots, like from Assembling 2 to 3.
- The slotted type of modules must be different during the upgrade, resulting in a removal of a module.

I'm basing that on the fact that only when I explicitly tell it to upgrade the modules it crashes. Save file for repro: https://share.xaymar.com/view/b03969a9-f048-4079-8e89-e4e64c527469 - just walk up and you should crash.

100875.403 Error MainLoop.cpp:1432: Exception at tick 168528635: The mod Spiderbots (0.2.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event spiderbots::on_spider_command_completed (ID 181)
Index out of inventory bounds.
stack traceback:
    [C]: in function '__index'
    __spiderbots__/control.lua:745: in function 'insert_items'
    __spiderbots__/control.lua:791: in function 'complete_task'
    __spiderbots__/control.lua:809: in function <__spiderbots__/control.lua:797>