Fix race condition if flutter version command returns too quickly in SDK settings

See context in https://github.com/flutter/flutter-intellij/pull/9016#discussion_r3476258845

I don't think this is very likely to happen, and I will certainly raise the priority if we see reports of this occurring.

Start with the discussion in flutter-intellij#9016, then trace the SDK settings path that runs the Flutter version command. Check the behavior when that command returns immediately and verify that the race no longer occurs while the issue remains open.

Written by the indexing model from the issue text.