Thanks @joffreyjaffeux for the fix 
The error was occurring from here: discourse-events/assets/javascripts/discourse/models/provider.js at b38f3258115110c07951f18e2c3562ac3e68a301 路 angusmcleod/discourse-events 路 GitHub
We deprecated the use of that Singleton mixin at the start of the year. This PR should provide some reference on how to refactor a class to use Singleton as a decorator.
@angus would you be able to take a look? It should be a fairly minimal change, the decorator can be applied on RestModel.