Just submitted a PR to fix this Discourse plugin tutorial:
The notebook tutorial had an error due to this issue:
Which I fixed and tested with the PR:
and it works again:
Hope this helps.
PS: Slowly, I'm making progress with learning to write Discourse plugins... very slowly 
See also:
And also on meta:
And if you want to clone the working version until the owner accepts the PR:
Just pushed another version of this tutorial (0.7.2) out to Github; adding code to log various Rails calls to a log file so people, like me, learning Discourse plugins and Rails can see how various parts of this plugin work:
See also (as before)
https://github.com/unixneo/notebook-neo
I plan to add more "learning tools" to his tutorial plugin over time and perhaps do a new write-up to complement the original tutorial, where I may focus on the Rails MVC concepts (still thinking about this, since I am still learning Rails).
Make a lot of changes to my fork:
# frozen_string_literal: true
# name: notebook
# about: notebook is a fork of the notebook tutorial plugin https://github.com/kleinfreund/notebook
# version: 0.7.3
# authors: kleinfreund, unixneo
# url: https://github.com/unixneo/notebook-neo