Error in AG Dashboard when looking at a Distributed AG
As a couple users mentioned in the comments, if you are not actually having problems with the AG, this is likely a bug with SSMS.  

You mentioned you're on 17.4, and I see in the release notes for 17.6 that there are several fixes for AG issues, including:


  Fixed an issue when the primary is down and manually failover to secondary, a NullReferenceException will be thrown.


(a NullReferenceException in .NET classically includes the message you mentioned - "Object reference not set to an instance of an object")

You should try updating to 17.6 (build 14.0.17230.0) to see if the problem is resolved there.