It appears that the AuthManager::singleton() method was deprecated in 1.35 and removed in 1.37.
I think I have fixed this now; please try the current dev-main version of this extension. (You can install it via composer, using “dev-main” as the version number.) It works for me on 1.35, and if it works for you in 1.37 then I will properly tag it as a bug fix release (e.g., 2.0.1).