For some reaosn, when we discovered our URI, we were re-instantiating
the `Completer` instance variable whose future we listen to in `nextUri()`.
This led to a race between a caller calling `nextUri()` and us discovering
the URI. If we happened to discover our URI before a caller called
`nextUri()`, then they would be left waiting on a future from the newly
allocated `Completer` (which would never complete).

Fixes #10064