The new version of Flow forced me to type this collection, which revealed a type error.
The issue is that Flow doesn't want us to call validProxies.has with a type that doesn't match the indexer. Since proxy is nullable, we first need to check if it's set before calling validProxies.has.
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 D9867