fixed type hint

app.py

@@ -34,7 +34,7 @@ def messages_to_history(messages: Messages) -> Tuple[str, History]:
         history.append([q['content'], r['content']])
     return system, history
 
-def model_chat(query: Optional[str], history: Optional[History]) -> Generator[Tuple[str, str, History]]:
+def model_chat(query: Optional[str], history: Optional[History]) -> Generator[Tuple[str, str, History], None, None]:
     if query is None:
         query = ''
     if history is None: