The latest commit on this plugin fixes an error for cutting edge 3.2 but breaks the plugin for stable 3.1.2. Is it possible to revert this commit until 3.2 is released? Or add a couple lines of code to make it work for both versions of Glyphs?
You can use the original instead of the fork, that should work in 3.1.
Funny, I stumbled across this exact issue today myself (but the other way round: I was trying to use the original plugin in 3.2).
Can you try again? I hope I fixed it.