Traceback error, also messes with colours

[randGit0]

Traceback (most recent call last):
File "C:\Program Files (x86)\Steam\steamapps\common\Blender\2.83\scripts\addons\material-combiner-addon\operators\combiner\combiner.py", line 52, in invoke
clear_duplicates(self.structure)
File "C:\Program Files (x86)\Steam\steamapps\common\Blender\2.83\scripts\addons\material-combiner-addon\operators\combiner\combiner_ops.py", line 104, in clear_duplicates
ob.data.materials.pop(index=mat_idx)
RuntimeError: Error: Index out of range

location: :-1

There are no actual "textures" in this, just purely flat materials with different colours, and metal/specular/roughness values, Use Nodes disabled. It messes with colours pretty hard.
I'll suppose I might as well learn how to UV map all of this so I can do it manually, at this point.

How it originally looks like

How it looks like (one of the variations depending on which ones I select, only selecting the materials for the buttons results in all of them merging into one colour

The error message, consistently appearing, whether CATS=true or false.