View with SentryOne Plan Explorer from SSMS context menu doesn't work if Plan Explorer is already open
Well, the best option is to just generate the plan from within SentryOne Plan Explorer in the first place, instead of switching back and forth. You get much richer metrics this way, because it captures a bunch of things SSMS doesn't, including actual costs, I/O, and Index Analysis.
But yes, this was a defect in an earlier release, that came about after the add-in had to change yet again due to the evolution of SSMS and how it works with add-ins.