Factorio version: 0.18.24
Rampant version: 0.18.11
Road Grid version: 0.0.3
Map evolution: not sure. I can provide a save file and factorio-previous.log. Probably unrelated.
Was Rampant added at the start of a game: yes.

I was walking north, outside of the starting area. I think I set starting area size to max, and probably as soon as I cross that "border" I think road grid wanted to add extra tiles and it caused a crash. Not sure if it it's reproducable or which of the two mods is at fault but I can provide a save.

38540.522 Error MainLoop.cpp:1205: Exception at tick 1101919: The mod Road Grid (0.0.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event road-grid::on_chunk_generated (ID 12)
The mod Rampant (0.18.11) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event Rampant::on_robot_built_tile (ID 47)
Rampant/control.lua:783: attempt to index field 'item' (a nil value)
stack traceback:
Rampant/control.lua:783: in function <Rampant/control.lua:779>
stack traceback:
road-grid/control.lua:26: in function <road-grid/control.lua:16>
stack traceback:
[C]: in function 'raise_event'
road-grid/control.lua:26: in function <road-grid/control.lua:16>
38545.876 Quitting: user-quit.

Save file: https://drive.google.com/drive/folders/1KL1r9kPQ83wAreEOzqX3BEi-eV7WLuqP?usp=sharing
Cross posted to https://mods.factorio.com/mod/road-grid/discussion/5ec424410863ce000dad87d1

I also shared some code insights. I'm thinking Rampant doesn't like on_robot_built_tile without the item or the item name.
That, or road grid should also attach item data so other mods can properly read what item has been placed.

I'm getting a similiar error with the Construction Drones and the Transport drones mod (I'm not using Road grid) and Rampant. Happened when placing road tiles with the drones. It also says:

Error while running event Rampant::on_robot_built_tile (ID 47)
Rampant/control.lua:783: attempt to index field 'item' (a nil value)

So it seems Rampant needs to protect against that somehow.

Btw, I love that mod, although I hadn't much chance to play with it, it feels great so far! That's how the AI should be :)

Since the code in rampant is only to check if and when landfill is placed it shouldn't cause bugs if a nil check is implemented. Assuming biters can walk over klonans roads that is. A fix is in the link from my Google drive.

The next version should have a fix for this issue.

Thank you for your hard work and amazing mod :)

Thank you, the latest should fix this.