Fix setting None update workers not causing all threads to stop

telethon/update_state.py

@@ -79,9 +79,12 @@ class UpdateState:
         """Raises "StopIterationException" on the worker threads to stop them,
            and also clears all of them off the list
         """
+        if self._workers:
             with self._updates_lock:
                 # Insert at the beginning so the very next poll causes an error
+                # on all the worker threads
                 # TODO Should this reset the pts and such?
+                for _ in range(self._workers):
                     self._updates.appendleft(StopIteration())
                 self._updates_available.set()