RingCentral 6.1.0 null object reference when making API call.

I'm converting a Winforms app written in 2016 to core using RingCentral.Net 6.1.0. I've studied GitHub code and only need Oauth like we've been doing since 2016. I can instantiate and add an extension successfully to the RestClient but when I make an API call I get a null object reference. Debugging the RestClient object the token is null. The exception message isn't clear what is null, except I can see the Token property of the RestClient is null. See below.