After a spot of head scratching and thinking that there was something wrong with my AG setup, it turns out that there’s a bug in SSMS. I was running SSMS 17.5 although this may well also affect earlier versions.
Looking at the release notes for SSMS 17.6, one of the bug fixes that this version addresses is…
Fixed an issue when the primary is down and manually failover to secondary, a NullReferenceException will be thrown.
David notes that upgrading fixed his issue; read on for more.