When I use whatsapp-web.js to send a message, there is a protocol error. here is the error information:

UnhandledPromiseRejectionWarning: Error: Protocol error (Runtime.callFunctionOn): Target closed.

const { Client } = require('whatsapp-web.js');
const qrcode = require('qrcode-terminal');
const client = new Client();
client.on('qr', (qr) => {
    qrcode.generate(qr, {small: true});
});

client.on('ready', async() => {
    console.log('Client is ready!');
    client.isRegisteredUser("xxxxxxx@c.us").then(function(isRegistered) {
        console.log('isregister......',isRegistered)
        if(isRegistered) {
            client.sendMessage("xxxxxx@c.us", "hello");
        }
    })  
});

client.on('message', msg => {
    console.log('msg.....',msg)
    if (msg.body == '!ping') {
        msg.reply('pong');
    }
});

client.initialize();