After typing the Map, Flow doesn't like that we're calling get with a value that is nullable. To address this I add an invariant that should never fire since we only show message results that are delivered from the server, which always have id set.
NOTE: CI will fail on this diff. I considered the possibility of fixing Flow errors BEFORE upgrading Flow, but it wasn't possible... in some cases, the fixes to support the new version of Flow caused errors in the old version. I could have hidden these type errors with $FlowFixMe lines and then later revert those, but that seemed like too much busy work.
Depends on D10028