Broken by Discord/BD updates. I recommend using JustOptimize's rewrite of this plugin's functionality instead: https://github.com/JustOptimize/return-ShowHiddenChannels
BetterDiscord Addons 
This is a maintained fork of DevilBro's repository for his BetterDiscord plugins and themes. It features an up-to-date version of the BDFDB library as well as a working version of the ShowHiddenChannels plugin.
Copyright (C) 2016-2022 Mirco Wittrien