Vehicle Grid (https://mods.factorio.com/mod/VehicleGrid) version 1.3.0 added support for spidertrons (included modded versions). The default spidertron grid size in that mod is set to 10x10 for all spidertrons without grids. This causes a crash in your mod when upgrading from MK2 to MK3 if equipment is added to the spidertron and not contained in a 5x5 (from topleft) square as the MK2 engineer is given a 10x10 grid by vehiclegrid (as it does not have a grid), but the MK3 is left at 5x5. This error can be circumvented by (as a user) setting the grid added by VehicleGrid to 4x4 for spidertrons, or by (this mod) using any size grid instead of nil.

Thanks for the report and workaround. I thought that I had added a fallback for when 'upgrading' to a spidertron with a smaller equipment grid, but I guess I never tested it, so it clearly doesn't work!

Do you think that I should override the added vehicle grid on MK1&2 spidertrons to remove them entirely? I think that makes the most logical sense.

Yeah that seems like the simplest solution.

Regardless, thanks for a very neat mod idea :D

Fixed in 1.6.1. Thanks for reporting!