How to handle exceptions for all the sub apps in FastAPI

I have a FastAPI project containing multiple sub apps (The sample includes just one sub app).

main_app = FastAPI()class CustomException(Exception):      def __init__(self, message: str, status_code: int, name: str = "Exception"):        Exception.__init__(self)        self.name = name        self.status_code = status_code        self.message = message@main_app.exception_handler(CustomException)async def custom_exception_handler(exception: CustomException) -> JSONResponse:    return JSONResponse(        status_code=exception.status_code, content={"error": exception.message}    )main_app.mount("/subapp", subapp1)  

I've handled the exceptions in main app, but not in subapp1. Now if I use the CustomException in subapp1:

raise CustomException(    status_code=status.HTTP_404_NOT_FOUND,    message=f"{self.model.__name__} not found",)

I get this error:

RuntimeError: Caught handled exception, but response already started.

It seems like when raising CustomException in a sub app, it won't be handled by the main app exception handler. So how can I handle the exceptions from all the sub app using the main app exception handler?