fixbug: an unexpected UserRequirement type message is thrown when there is nothing to do.

Author: 莘权 马

metagpt/roles/role.py

@@ -418,7 +418,7 @@ async def _react(self) -> Message:
         Use llm to select actions in _think dynamically
         """
         actions_taken = 0
-        rsp = Message(content="No actions taken yet")  # will be overwritten after Role _act
+        rsp = Message(content="No actions taken yet", cause_by=Action)  # will be overwritten after Role _act
         while actions_taken < self.rc.max_react_loop:
             # think
             await self._think()