我有一个Next.js（React）客户端，我已经设置它来监听来自我的Node.js/Express.js服务器的Server-Sent Events，但由于某种原因，它似乎没有接收消息。
EventSource的open和error事件工作正常，但每当我使用.onmessage时，它都检测不到任何东西。
我做错了什么？
Express.js代码：

router.get('/stream/:walletAddress', async (req, res, next) => {
    try {
        res.set({
            Connection: 'keep-alive',
            'Content-Type': 'text/event-stream',
            'Cache-Control': 'no-cache',
            'Access-Control-Allow-Origin': '*'
        });
        res.flushHeaders();

        let x = 0;
        const id = setInterval(() => {
            res.write(`event: message\n`);
            res.write(`data: ${x++}\n\n`);
        }, 2000);

        res.on('close', () => {
            console.log('Client closed.');
            clearInterval(id);
        });
    } catch (err) {
        next(err);
    }
});

Next.js（React）代码：

import { API_URL } from 'config';
import { useWalletStore } from 'src/stores';

export const FetchTxsProvider = ({ children }) => {
    const safeSdkReader = useWalletStore(state => state.safeSdkReader);

    useEffect(() => {
        const eventSource = new EventSource(
            `${API_URL}/transactions/stream/${safeSdkReader.getAddress()}`
        );

        eventSource.onopen = function (e) {
            console.log('Successfully connected.');
        };

        eventSource.onerror = function (err) {
            console.error(err);
            eventSource.close();
        };

        // ! Should fire here but doesn't!
        eventSource.onmessage = function (event) {
            console.log('Event: ', event);
        };

        return () => {
            console.log('Connection closed.');
            eventSource.close();
        };
    }, [safeSdkReader]);

    return <>{children}</>
};