remove default trigger value for register_nested_chats

autogen/agentchat/conversable_agent.py

@@ -362,15 +362,15 @@ def _summary_from_nested_chats(
     def register_nested_chats(
         self,
         chat_queue: List[Dict[str, Any]],
-        trigger: Union[Type[Agent], str, Agent, Callable[[Agent], bool], List] = [Agent, None],
+        trigger: Union[Type[Agent], str, Agent, Callable[[Agent], bool], List],
         reply_func_from_nested_chats: Union[str, Callable] = "summary_from_nested_chats",
         position: int = 2,
         **kwargs,
     ) -> None:
         """Register a nested chat reply function.
         Args:
             chat_queue (list): a list of chat objects to be initiated.
-            trigger (Agent class, str, Agent instance, callable, or list): Default to [Agent, None]. Ref to `register_reply` for details.
+            trigger (Agent class, str, Agent instance, callable, or list): refer to `register_reply` for details.
             reply_func_from_nested_chats (Callable, str): the reply function for the nested chat.
                 The function takes a chat_queue for nested chat, recipient agent, a list of messages, a sender agent and a config as input and returns a reply message.
                 Default to "summary_from_nested_chats", which corresponds to a built-in reply function that get summary from the nested chat_queue.