Unable to open a web browser (URL) from my Flutter code

Question

I am building a Flutter app, and I'd like to open a URL into a web browser or browser window (in response to a button tap). But I am unable to open it.

This my code-

import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';

@override
  Widget build(BuildContext context) {
    return Scaffold(
    body: Center(
      child: ElevatedButton(
        onPressed: _launchURL,
        child: new Text('FLutter'),
      ),
    ),
  ));

_launchURL() async {
  const url = 'https://flutter.io';
  final uri = Uri.parse(url);
  if (await canLaunchUrl(uri)) {
    await launchUrl(uri);
  } else {
    throw 'Could not launch $url';
  }
}