RLlib: Fix bug: WorkerSet.stop() will raise error if self._local_worker is None (e.g. in evaluation worker sets). (#25332)

rllib/evaluation/worker_set.py

@@ -342,7 +342,8 @@ class WorkerSet:
     def stop(self) -> None:
         """Calls `stop` on all rollout workers (including the local one)."""
         try:
-            self.local_worker().stop()
+            if self.local_worker():
+                self.local_worker().stop()
             tids = [w.stop.remote() for w in self.remote_workers()]
             ray.get(tids)
         except Exception: