The problem occurs when integrating Firebase into Flutter Web.

Firebase cannot be initialized within Flutter Web.

The console error is shown below.

The main and index source files are below.

Error:
FirebaseError: Firebase: No Firebase App '[DEFAULT]' has been created - call initializeApp() first (app/no-app). https://www.gstatic.com/firebasejs/12.9.0/firebase-app.js 2270:29 getApp package:firebase_core_web/src/interop/core.dart 60:28 app$ package:firebase_core_web/src/firebase_core_web.dart 299:24 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 623:19 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 648:23 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 594:19 dart-sdk/lib/async/zone.dart 1849:54 runUnary dart-sdk/lib/async/future_impl.dart 222:18 handleValue dart-sdk/lib/async/future_impl.dart 948:44 handleValueCallback dart-sdk/lib/async/future_impl.dart 977:13 _propagateToListeners dart-sdk/lib/async/future_impl.dart 720:5 [_completeWithValue] dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 504:7 complete dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 571:12 _asyncReturn package:firebase_core_web/src/firebase_core_web.dart 225:3 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 623:19 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 648:23 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 594:19 dart-sdk/lib/async/zone.dart 1849:54 runUnary dart-sdk/lib/async/future_impl.dart 222:18 handleValue dart-sdk/lib/async/future_impl.dart 948:44 handleValueCallback dart-sdk/lib/async/future_impl.dart 977:13 _propagateToListeners dart-sdk/lib/async/future_impl.dart 720:5 [_completeWithValue] dart-sdk/lib/async/future.dart 533:22 dart-sdk/lib/async/zone.dart 1849:54 runUnary dart-sdk/lib/async/future_impl.dart 222:18 handleValue dart-sdk/lib/async/future_impl.dart 948:44 handleValueCallback dart-sdk/lib/async/future_impl.dart 977:13 _propagateToListeners dart-sdk/lib/async/future_impl.dart 720:5 [_completeWithValue] dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 504:7 complete dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 571:12 _asyncReturn package:firebase_core_web/src/firebase_core_web.dart 197:3 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 623:19 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 648:23 dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 594:19 dart-sdk/lib/async/zone.dart 1849:54 runUnary dart-sdk/lib/async/future_impl.dart 222:18 handleValue dart-sdk/lib/async/future_impl.dart 948:44 handleValueCallback dart-sdk/lib/async/future_impl.dart 977:13 _propagateToListeners dart-sdk/lib/async/future_impl.dart 720:5 [_completeWithValue] dart-sdk/lib/async/future_impl.dart 804:7 dart-sdk/lib/async/schedule_microtask.dart 40:34 _microtaskLoop dart-sdk/lib/async/schedule_microtask.dart 49:5 _startMicrotaskLoop dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 118:77 tear dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 188:69

main.dart:

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  await Firebase.initializeApp(
    options: const FirebaseOptions(
      apiKey: "xxx",
      authDomain: "xxx",
      projectId: "xxx",
      storageBucket: "xxx",
      messagingSenderId: "xxx",
      appId: "xxx",
      measurementId: "xxx",
    ),
  );

  runApp(const MyApp());
}

index.html: