When focus leaves the To field while the address DB query is in flight,
the optionsBuilder Future completes AFTER RawAutocomplete has already
called hide() on the overlay. The completion triggers a second hide()
call, hitting the _zOrderIndex != null assertion in overlay.dart.
Fix: check focusNode.hasFocus after the await; return [] if focus left,
which prevents RawAutocomplete from calling show()/hide() on a closed
overlay.
Also fixes#81 partially: after undo(), push an inverse UndoAction so
the undo log retains a record and the user can re-apply the operation.
Fixes#79
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
