hello again 
i tried it and unfortunately it do not work for me 
i guess i have to trie the files from github which are posted from mariopeper
I am not sure why you are getting this this error message. Did you just activeted the pre-installed Material Utilities add-on?
My guess is that your appdata has something to do with this. Maybe you could wipe your add-on folder under C:\Users\YourWindowsUserName\AppData\Roaming\Blender Foundation\Blender\2.82\scripts\addons and reinstall blender let me know if it works for you.
(2.93) Select the object, go to the material tab, below the + and - is a drop down arrow with more options
select delete all unused slots.
Then select all the objects you want to do the same and press shift + R and blender will reapeat the last action (delete unused slots) to all of the selected objects 
I hope it works
Wrote a simple script to make the process quick. This was written for my personal use on a certain project. All you have to do is to simply drag and drop the file into Blender Scripting tab and hit play
Script:
Note: This can be used to remove all materials including material slots
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Thank you! Thank you!!
added a version 4.x.x compatible version as pull request to