I have a fully functional App on Android and IOS, and now I want to have a web version taking advantage of Flutter's cross-platform features.

To do this, I created a "Chrome Device" from VS Code and I did the Firebase App registration within the Firebase console. In the index.html I included the configuration as explained in Flutter Firebase Installation Web ...

<!DOCTYPE html>
<html>
<head>
  <!--
    If you are serving your web app in a path other than the root, change the
    href value below to reflect the base path you are serving from.

    The path provided below has to start and end with a slash "/" in order for
    it to work correctly.

    For more details:
    * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base

    This is a placeholder for base href that will be replaced by the value of
    the `--base-href` argument provided to `flutter build`.
  -->
  <base href="$FLUTTER_BASE_HREF">

  <meta charset="UTF-8">
  <meta content="IE=Edge" http-equiv="X-UA-Compatible">
  <meta name="description" content="A new Flutter project.">

  <!-- iOS meta tags & icons -->
  <meta name="apple-mobile-web-app-capable" content="yes">
  <meta name="apple-mobile-web-app-status-bar-style" content="black">
  <meta name="apple-mobile-web-app-title" content="appname">
  <link rel="apple-touch-icon" href="icons/Icon-192.png">

  <title>AppName</title>
  <link rel="manifest" href="manifest.json">
</head>
<body>

  <script src="https://www.gstatic.com/firebasejs/8.6.1/firebase-app.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.6.1/firebase-auth.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.6.1/firebase-database.js"></script>
  <script src="https://www.gstatic.com/firebasejs/8.6.1/firebase-messaging.js"></script>

  <script>

    // Your web app's Firebase configuration
    // For Firebase JS SDK v7.20.0 and later, measurementId is optional
    var firebaseConfig = {
      apiKey: "xxx", //with my particular values
      authDomain: "xxx",
      databaseURL: "xxx",
      projectId: "xxx",
      storageBucket: "xxx",
      messagingSenderId: "xxx",
      appId: "xxx",
      measurementId: "xxx"
    };

    // Initialize Firebase
   firebase.initializeApp(firebaseConfig); //
  </script>

  <!-- This script installs service_worker.js to provide PWA functionality to
       application. For more information, see:
       https://developers.google.com/web/fundamentals/primers/service-workers -->
  <script>
    var serviceWorkerVersion = null;
    var scriptLoaded = false;
    function loadMainDartJs() {
      if (scriptLoaded) {
        return;
      }
      scriptLoaded = true;
      var scriptTag = document.createElement('script');
      scriptTag.src = 'main.dart.js';
      scriptTag.type = 'application/javascript';
      document.body.append(scriptTag);
    }

    if ('serviceWorker' in navigator) {
      // Service workers are supported. Use them.
      window.addEventListener('load', function () {
        // Wait for registration to finish before dropping the <script> tag.
        // Otherwise, the browser will load the script multiple times,
        // potentially different versions.
        var serviceWorkerUrl = 'flutter_service_worker.js?v=' + serviceWorkerVersion;
        navigator.serviceWorker.register(serviceWorkerUrl)
          .then((reg) => {
            function waitForActivation(serviceWorker) {
              serviceWorker.addEventListener('statechange', () => {
                if (serviceWorker.state == 'activated') {
                  console.log('Installed new service worker.');
                  loadMainDartJs();
                }
              });
            }
            if (!reg.active && (reg.installing || reg.waiting)) {
              // No active web worker and we have installed or are installing
              // one for the first time. Simply wait for it to activate.
              waitForActivation(reg.installing || reg.waiting);
            } else if (!reg.active.scriptURL.endsWith(serviceWorkerVersion)) {
              // When the app updates the serviceWorkerVersion changes, so we
              // need to ask the service worker to update.
              console.log('New service worker available.');
              reg.update();
              waitForActivation(reg.installing);
            } else {
              // Existing service worker is still good.
              console.log('Loading app from service worker.');
              loadMainDartJs();
            }
          });

        // If service worker doesn't succeed in a reasonable amount of time,
        // fallback to plaint <script> tag.
        setTimeout(() => {
          if (!scriptLoaded) {
            console.warn(
              'Failed to load app from service worker. Falling back to plain <script> tag.',
            );
            loadMainDartJs();
          }
        }, 4000);
      });
    } else {
      // Service workers not supported. Just drop the <script> tag.
      loadMainDartJs();
    }
  </script>

</body>
</html>

When I run the App in Chrome I get an Oh No! Error 5 in the browser and no error messages in the VS Code console.

If I include an await in the Firebase initialization line in index.html:

await firebase.initializeApp(firebaseConfig);

Again the Oh No! Error 5, but when I reload the page I get the following error on web_entrypoint.dart:

FirebaseError: Firebase: No Firebase App '[DEFAULT]' has been created - call Firebase App.initializeApp() (app/no-app). at Object.u [as app] (https://www.gstatic.com/firebasejs/8.6.1/firebase-app.js:1:18836) at Object.getApp (http://localhost:65003/packages/firebase_database_web/src/interop/app.dart.lib.js:630:89) //... more debug lines

In my lib/src/pages/main.dart, which works fine on IOS and Android, I'm making sure to initialize the Firebase services first ...

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  final prefs = PreferenciasUsuario();
  await prefs.initPrefs();
  await PushNotificationsProvider.initializeApp();
  FirebaseDatabase database;
  database = FirebaseDatabase.instance;
  database.setPersistenceEnabled(true);
  database.setPersistenceCacheSizeBytes(10000000); 
  runApp(MyApp());
}

I've read most of the answers on the subject on StackOverflow, but they haven't worked for me, I don't know if it's because I'm doing it particularly with Flutter.

How can I resolve the No Firebase App '[DEFAULT]' error and get my application run on the web?