Flet广播消息无法接收怎么办？

async def main(page: ft.Page):    page.title = "Flet Chat"    async def on_message_async(msg):        messages.controls.append(ft.Text(msg))        await page.update_async()    await page.pubsub.subscribe_async(on_message_async)    async def send_click(e):        await page.pubsub.send_all_async(f"{message.value}")        message.value = ""        await page.update_async()    messages = ft.Column()    message = ft.TextField(hint_text="Your message...", expand=True)    send = ft.ElevatedButton("Send", on_click=send_click)    await page.add_async(messages, ft.Row(controls=[message, send]))