How to fix firebase database initialised multiple times due to React SSR initialised database and cloud function firebase initialised database?

I have updated the question as found the root cause of the issue. As I have hosted my React SSR app which uses firebase database in the client serving by one of the cloud function named app throwing an error of Error: FIREBASE FATAL ERROR: Database initialized multiple times. Please make sure the format of the database URL matches with each database() call.. When I comment out one by one and deploy, works perfectly. But when I deploy together doesn't work. How do I separate these two keeping both at the same repo?

ORIGINAL Question: Why firebase cloud function throwing an error of 'The default Firebase app does not exist.'? So I am trying out firebase function for the first time. admin.messaging() throwing me the following error. Help me figure out why? If I look at the console I get results till console.log('deviceToken', deviceToken);

so whats wrong in const messageDone = await admin.messaging().sendToDevice(deviceToken, payload);?

const functions = require('firebase-functions');
const admin = require('firebase-admin');
exports.updateUnreadCount = functions.database.ref('/chats/{chatId}/{messageId}')
  .onCreate(async(snap, context) => {
    const appOptions = JSON.parse(process.env.FIREBASE_CONFIG);
    appOptions.databaseAuthVariableOverride = context.auth;
    const adminApp = admin.initializeApp(appOptions, 'app');
    const { message, senderId, receiverUid } = snap.val();
    console.log(message, senderId, receiverUid);
    console.log('------------------------');
    const deleteApp = () => adminApp.delete().catch(() => null);
    try {
      const db = adminApp.database();
      const reciverUserRef = await db.ref(`users/${receiverUid}/contacts/${senderId}/`);
      console.log('reciverUserRef', reciverUserRef);
      const deviceTokenSnapshot = await reciverUserRef.child('deviceToken').once('value');
      const deviceToken = await deviceTokenSnapshot.val();
      console.log('deviceToken', deviceToken);

      const payload = {
        notification: {
          title: 'Test Notification Title',
          body: message,
          sound: 'default',
          badge: '1'
        }
      };
      const messageDone = await admin.messaging().sendToDevice(deviceToken, payload);
      console.log('Successfully sent message: ', JSON.stringify(messageDone));
      return deleteApp().then(() => res);
    } catch (err) {
      console.log('error', err);
      return deleteApp().then(() => Promise.reject(err));
    }
  });

Update1: According to this https://firebase.google.com/docs/cloud-messaging/send-message#send_to_a_topic, admin.messaging().sendToDevice(deviceToken, payload) APIs are only available in the Admin Node.js SDK? So switched to

const payload = {
        data: {
          title: 'Test Notification Title',
          body: message,
          sound: 'default',
          badge: '1'
        },
        token: deviceToken
      };
      const messageDone = await admin.messaging().send(payload);

Which is not working either. Getting an error Error: The default Firebase app does not exist. Make sure you call initializeApp() before using any of the Firebase services. Any lead will be helpful.

EDIT: Finally got the function working. My index.js is exporting to functions, follwoing

exports.app = functions.https.onRequest(app); //React SSR
exports.updateChat = functions.database.ref('/chats/{chatId}/{messageId}').onCreate(updateChat);

exports.app is a react ssr function, which I am using to host my site. This uses database too. and throwing error of multiple database instance. When I comment out one by one and deploy, works perfectly. But when I deploy together doesn't work. How do I separate these two keeping both at the same repo? Any suggestions, please?