"Cannot read property 'PROVIDER_ID' of undefined" error on version 2.1.1

For some reason, when clicking on any button results on this JavaScript error

firebaseui.js:331 Uncaught TypeError: Cannot read property 'PROVIDER_ID' of undefined
    at firebaseui.js:331
    at b (firebaseui.js:331)
    at firebaseui.js:73
    at ol.<anonymous> (firebaseui.js:273)
    at cf (firebaseui.js:148)
    at Bg.h.dispatchEvent (firebaseui.js:147)
    at Dg (firebaseui.js:184)
    at Bg.Tc (firebaseui.js:183)
    at af (firebaseui.js:144)
    at HTMLButtonElement.We (firebaseui.js:145)

My initialization code:

<script>
          var config = window.settings.FirebaseCredentials;
          firebase.initializeApp(config);
</script>
<script src="https://cdn.firebase.com/libs/firebaseui/2.1.1/firebaseui.js"></script>
<script>

   var uiConfig = {
        signInSuccessUrl: "/",
        signInOptions: [
            // Leave the lines as is for the providers you want to offer your users.
            firebase.auth.GoogleAuthProvider.PROVIDER_ID,
            firebase.auth.GithubAuthProvider.PROVIDER_ID,
            firebase.auth.TwitterAuthProvider.PROVIDER_ID,
            firebase.auth.FacebookAuthProvider.PROVIDER_ID
        ],
        tosUrl: "http://howlbits.com"
    };

    // Initialize the FirebaseUI Widget using Firebase.
    var ui = new firebaseui.auth.AuthUI(firebase.auth());
    // The start method will wait until the DOM is loaded.
    ui.start('#firebase-auth-container', uiConfig);
...

[shaktikathpalia]

Adding <script src="https://www.gstatic.com/firebasejs/6.3.2/firebase-auth.js"></script> worked for me

[ChrisBlueSpurs]

Thank you! This did lead to a solution but interestingly, it was nothing to do with firebaseAuthObj and everything to do with importing firebase and firebaseui. I had used

let fire = require ('firebase');
let firebaseui = require('firebaseui');

let config = { ... };

let firebase = fire.initializeApp(config);

Changing this simply to:

import firebase from 'firebase';
import firebaseui from 'firebase';

let config = { ... };

firebase.initializeApp(config);

Made it work. Interesting that the way I was doing it actually gave me working access to cloud firestore, but problems with firebase auth. Anyway, thanks very much for the help, still fairly new to React and ES6. Hopefully this can help someone else if they ever come across it.