The new version of Flow detected a slight type error here. lastID is nullable, but the type for useSearchMessages requires cursor to be string if specified. We can address this by making cursor nullable as well.
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 D10027