THIS CHANGES SERVER BEHAVIOUR (intentionally); no wire change.
The MissionDialogueOK handler (carried over verbatim from
GameMessages::HandleMissionDialogOK) dereferenced the responder without
a null check, so a client naming an object that doesn't exist crashed
the world server. It now logs and ignores the message; the script
callback is no longer called with a null player (that path always
ended in the crash).
Test: handling a MissionDialogueOK with an unknown responder crashes
without this change and passes with it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>