Fix n might be None when stopping workers

telethon/update_state.py

@@ -71,6 +71,7 @@ class UpdateState:
         # Put dummy ``None`` objects so that they don't need to timeout.
         n = self._workers
         self._workers = None
+        if n:
             with self._updates_lock:
                 for _ in range(n):
                     self._updates.put(None)