I had a crash when my bots ghost built a Tiberium Spike, and the code at control.lua line 777 tried to index the surface of the entity, it ended up being a nil and failing. You should probably take steps provide an escape path for if it ends up being invalid when you're doing your thing.

Do you have other routines in your mod, like destroying the entity that was built by the robot and placing another instead ? Anyway, i will test entity valid on the next version. Thanks for the info

It's not my mod.

Ok. update released.

Thanks for the quick fix.
Also, since you are binding On_Built to the script_raised_built event, you will need to have entity = event.created_entity or event.entity because script_raised_built's event doesn't have a created_entity: https://lua-api.factorio.com/latest/events.html#script_raised_built

Ohh you are right. I missed that, thanks. Fixing...