Sorry I cannot comment on your question because my rank is too low. I had the same error coming up when I followed the react version of you tutorial.
The issue in my case was due to the fact that in the auth.js file, there is a function called getTokenExpirationDate(encodedToken). Inside that function the line const token = decode(encodedToken); was the cause of the bug.
By investigating the value of encodedToken I found out that it's value was a string with the characters 'none', (not the value none itself!). This makes the encode function very unhappy.
If you go in the chrome developer tools, click on the Application tab, open the Local Storage option on the left hand side and select you site. You will be able to see all the key-value pairs stored in your local storage for your app. In my case the key id_token had the value none. If it's the case for you too, just delete that key/value pair from the local storage and the page should be able to load again.
Hope this helped and good luck!
Edit
By playing with local storage with chrome developer tools I realised the local storage can be a bit buggy. Sometimes I try to view the local storage but it doesn't appear. When that happens just try closing and reopening the tool.