What version of FatController and Factorio? I'm using 0.0.9/0.11.8 without problems.
My control.lua line 42 looks like this:
glob.fatControllerButtons = game.players[1].gui.top.add({ type="flow", name="fatControllerButtons", direction="horizontal", style="fatcontroller_thin_flow"})
if yours is ...

I'm getting the following:
Any advice how to fix it? Crashes the game when I load. Everything was working fine and then I started getting it all the sudden. If I disable the mod in-game the game runs fine, but obviously Fat Controller doesn't work then. Thanks!