from origin 'null' has been blocked by cors policy

const express = require('express');
const app = express();

app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, OPTIONS');
  res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  next();
});

app.get('/data', (req, res) => {
  res.json({ message: 'This is data from the server.' });
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

from flask import Flask, jsonify

app = Flask(__name__)

@app.route('/data')
def get_data():
    response = jsonify({'message': 'This is data from the server.'})
    response.headers.add('Access-Control-Allow-Origin', '*')
    response.headers.add('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE, OPTIONS')
    response.headers.add('Access-Control-Allow-Headers', 'Content-Type, Authorization')
    return response

if __name__ == '__main__':
    app.run(port=3000)

import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class DataController {

    @CrossOrigin(origins = "*")
    @GetMapping("/data")
    public String getData() {
        return "{\"message\": \"This is data from the server.\"}";
    }
}