Writing any code in Modmain results in meaningless crash

[NeddoFreddo]

Hi,

Very stuck on working on my mod at the moment.

Whenever I add any new code to modmain.lua, I get this frustratingly unhelpful error:

[00:00:02]: [string "scripts/mainfunctions.lua"]:1596: variable 'SetGlobalErrorWidget' is not declared

(Full log is attached).
I was originally trying to add controller support to my mod by calling 

GLOBAL.TheInput:AddControlHandler

which resulted in this crash, making me think that that line of code was the cause, until I tried adding a custom pickup sound for some of my items by calling table.insert which gave the same crash again.

This makes me think that there must be something else at play considering I am literally copying the controller code from another mod.

FYI this mod is 'finished,' I'm just trying to fix bugs and add controller support at this stage, there is already plenty of code in modmain.lua that can run no problem.

master_server_log.txt