The script has an unsupported MIME type ('text/plain').

I'm trying to get the registered_token, but I keep getting the following error.
How can I fix it?

index.html

```

A function is triggerled when the button is clicked. The function outputs some text in a p element with id="demo".

var firebaseConfig = {
apiKey: "AIzaSyBfLYQieTSNFJVghLmwPidH9eMLJ8sgafA",
authDomain: "push-server-56a52.firebaseapp.com",
databaseURL: "https://push-server-56a52.firebaseio.com",
projectId: "push-server-56a52",
storageBucket: "push-server-56a52.appspot.com",
messagingSenderId: "967213415011",
appId: "AIzaSyBfLYQieTSNFJVghLmwPidH9eMLJ8sgafA",
measurementId: "G-MDVS63TX7X"
};
// Initialize Firebase
firebase.initializeApp(firebaseConfig);

//모든 메세지 서비스 접근 ㅏ능
const messagings = firebase.messaging();
messagings.requestPermission()
.then(function() {
console.log('Have Permission');
return messagings.getToken();
})
.then(function(token){
console.log(token);
})
.catch(function(err){
console.log('Error Occured.',err);
})

// [END on_background_message]

```

firebase-messaging-sw.js
```
var firebaseConfig = {
apiKey: "AIzaSyBfLYQieTSNFJVghLmwPidH9eMLJ8sgafA",
authDomain: "push-server-56a52.firebaseapp.com",
databaseURL: "https://push-server-56a52.firebaseio.com",
projectId: "push-server-56a52",
storageBucket: "push-server-56a52.appspot.com",
messagingSenderId: "967213415011",
appId: "1:967213415011:web:ad877f78d7eff7212dd656",
measurementId: "G-MDVS63TX7X"
};
// Initialize Firebase
firebase.initializeApp(firebaseConfig);

const messaging = firebase.messaging();
messaging.usePublicVapidKey("BGgFrjLPNrLqke1o7vPLZPcrOGjfPeFVq82N3fzwtRjd37j9LuWc1xxmV7vXhrDpqFK_dgfuznGDRXVzC3fh1Hs")
messaging.getToken().then((currentToken) => {
if (currentToken) {
sendTokenToServer(currentToken);
updateUIForPushEnabled(currentToken);
} else {
// Show permission request.
console.log('No Instance ID token available. Request permission to generate one.');
// Show permission UI.
updateUIForPushPermissionRequired();
setTokenSentToServer(false);
}
}).catch((err) => {
console.log('An error occurred while retrieving token. ', err);
showToken('Error retrieving Instance ID token. ', err);
setTokenSentToServer(false);
});

```