Concurrency: can ref registry before initialized

See https://github.com/elementary/calendar/issues/742#issuecomment-1095716346:
> I believe I've tracked down the issue, and I don't think it's your fault, so I won't consider it a blocker for this PR. It looks like you uncovered a race condition. It appears that on constructing a `CalendarChooser` from a cold start, the `registry` property of the `EventStore` is null. From my testing, it seems like the error occurs before the registry is initialized in the async `open` method.

This causes a segfault because you're trying to call a method on a null object. To test, install a version that includes the updates in #742, which did something to reveal this issue.