问TypeError：无法读取exports.sendNotification.functions.database.ref.onWrite.event上未定义的属性“子”

EN

const functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp(functions.config().firebase);

exports.sendNotification = functions.database.ref('/Lecture_Materials/{MIS}/{MISId}')
    .onWrite(event => {

    // Grab the current value of what was written to the Realtime Database.
    var eventSnapshot = event.data;
    var str1 = "Lecture note uploaded is: ";
    var str = str1.concat(eventSnapshot.child("name").val());
    console.log(str);

    var topic = "Management.Information.System";
    var payload = {
        data: {
            name: eventSnapshot.child("name").val(),
        }
    };

    // Send a message to devices subscribed to the provided topic.
    return admin.messaging().sendToTopic(topic, payload)
        .then(function (response) {
            // See the MessagingTopicResponse reference documentation for the
            // contents of response.
            console.log("Successfully sent message:", response);
            return;
        })
        .catch(function (error) {
            console.log("Error sending message:", error);
        });
    });