fix: prevent get_user_ids_from_room crash on missing session (#20271)

Add null check in list comprehension before accessing session['id']. When a session_id exists in the room but has been removed from SESSION_POOL, the function now skips it instead of crashing with TypeError.

backend/open_webui/socket/main.py

@@ -246,7 +246,11 @@ def get_user_ids_from_room(room):
     active_session_ids = get_session_ids_from_room(room)
 
     active_user_ids = list(
-        set([SESSION_POOL.get(session_id)["id"] for session_id in active_session_ids])
+        set([
+            SESSION_POOL.get(session_id)["id"]
+            for session_id in active_session_ids
+            if SESSION_POOL.get(session_id) is not None
+        ])
     )
     return active_user_ids