Is this something that can be fixed in a future Xcode beta release, or will we have to install the latest MacOS beta? PreviewProvider produces a separate error 'Fatal Error in SchemaProperty.swift'.

Unfortunately when I try to kick off a new task, I get the error: … This object's deinit, or something called from it, may have created a strong reference to self which outlived deinit, resulting in a dangling reference.

I’m able to see the exercises in the Fitness app along with their times, but the green ring doesn’t fill up.

I misread the SE-0371 proposal at the time you posted. I see now it's a 'synchronous' deinit, so I don't see how it helps the situation of cleaning up with an asynchronous function. I tried it out yesterday in the release toolchain, and it didn't change the situation.