Firebase cloud message/notifications are not receiving. JS

I am trying to send FCM to my react app using cloud functions. Cloud function is executing, but notification is not being received on client side.

Here cloud function code.

exports.sendPush = functions.database.ref('/settings2').onWrite(event => {
    let projectStateChanged = false;
    let projectCreated = false;
    let projectData = event.data.val();
    if (!event.data.previous.exists()) {
        projectCreated = true;
    }
    if (!projectCreated && event.data.changed()) {
        projectStateChanged = true;
    }
    let msg = 'A project state was changed';
        if (projectCreated) {
            msg = `The following new project was added to the project: ${projectData.title}`;
        }
    tokens.push("fIGxxxxGtDGxxxx DEVICE Token");
    let payload = {
        notification: {
            title: 'Firebase Notification',
            body: 'This Is Message',
            sound: 'default',
            badge: '1'
        }
    };
    return admin.messaging().sendToDevice(tokens, payload);
});

Here is the log of Cloud function

And here is code on my client side:

messaging.onMessage(function(payload) {
   console.log("Message received. ", payload);
      // ...
});

There are not any errors, but nothing is in console. Do I have to do something with service worker file?

Any help would be appreciated.