FCM Web push notification unable to register the default service worker

I am using FCM web app notification for send notification to browser. **I Got The following errors :

• 1.A bad HTTP response code (404) was received when fetching the script. ------ firebase-messaging-sw.js
• 2..Failed to load resource: net::ERR_INVALID_RESPONSE
• 3.Error -> messaging/failed-serviceworker-registration
• 4.Error Message -> FirebaseError: Messaging: We are unable to register the default service worker. Failed to register a ServiceWorker: A bad HTTP response code (404) was received when fetching the script. (messaging/failed-serviceworker-registration).↵ at http://ift.tt/2ivJgjF↵ at **

My code below.

<script src="http://ift.tt/2h4CcKt"></script>
<script src="http://ift.tt/2iutXHH"></script>
<script src="http://ift.tt/2h4CdOx"></script>
<script>
  // Initialize Firebase
  var config = {
    apiKey: "APIKEYXXXXXXXXXXXXXXXXXXXXX",
    authDomain: "fir-cloudmessaging-d3607.firebaseapp.com",
    databaseURL: "http://ift.tt/2iv4ON2",
    projectId: "fir-cloudmessaging-d3607",
    storageBucket: "fir-cloudmessaging-d3607.appspot.com",
    messagingSenderId: "SENDERID123456789"
  };
  firebase.initializeApp(config);
</script>

<script>

  if ('serviceWorker' in navigator) {
  window.addEventListener('load', function() {
    navigator.serviceWorker.register('firebase-messaging-sw.js').then(function(registration) {
      // Registration was successful
      console.log('ServiceWorker registration successful with scope: ', registration.scope);
    }, function(err) {
      // registration failed :(
      console.log('ServiceWorker registration failed: ', err);
    });
  });
}
const messaging = firebase.messaging();
messaging.requestPermission()
.then(function() {
  console.log('Notification permission granted.');
  // TODO(developer): Retrieve an Instance ID token for use with FCM.
  // ...
  console.log(messaging.getToken());
 // return messaging.getToken();
})
.then(function(token){
  console.log(token);
})
.catch(function(err) {
  console.log('Unable to get permission to notify.', err);
});
</script>