Cannot read properties of null (reading 'split')

I was deploying my mern stack app to heroku, getting some kind of errors screenshot attached.

errors from activity logs: 2022-09-08T22:02:02.914114+00:00 app[web.1]: app is running at 35635 2022-09-08T22:02:02.914778+00:00 app[web.1]: Cannot read properties of null (reading 'split') 2022-09-08T22:02:03.072495+00:00 heroku[web.1]: Process exited with status 1 2022-09-08T22:02:03.137611+00:00 heroku[web.1]: State changed from starting to crashed 2022-09-08T22:06:33.637723+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/api/products" host=spiffy-shirts.herokuapp.com request_id=c6a06345-ab6b-4dbf-a76b-212dc9d9e81e fwd="27.4.164.92" dyno= connect= service= status=503 bytes= protocol=https

In my local env everything works fine.

My server index file code

// dependancy included: express,mongosse,cors,bodyParser, cookieParser ...
const app = express();

//DB
const connectDB = async () => {
  try {
    await mongoose.connect(process.env.MONGODB_URI, {
      useNewUrlParser: true,
    });

    console.log('MongoDB is Connected...');
  } catch (err) {
    console.error(err.message);
    process.exit(1);
  }
};
connectDB();

//My routes
const authRoutes = require('./routes/auth');

//Middlewares
//bodyparser, cookieparser,cors ...

//PORT
const port = process.env.PORT || 6000;

//My routes
app.use('/api', authRoutes);

if (process.env.NODE_ENV === 'production') {
  app.use(express.static('frontend/build'));
}

//Starting a Server
app.listen(port, () => {
  console.log(`app is running at ${port}`);
});