Why does Firebase.initializeApp() fail to create the '[DEFAULT]' Firebase App in Flutter release builds?

Below is a list of areas to check and adjust in your project when you see the error in release builds:

1. Ensure Firebase.initializeApp() Is Called

Make sure your Flutter app calls Firebase.initializeApp() before any Firebase functionality is used. For example, your main() should look similar to:

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void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

2. Configure google-services.json for All Flavors

Since you’re using flavors (e.g. prod), verify that your release flavor (prod) has the correct google-services.json file. Ensure that:

• The file is placed in the correct location for your flavor.
• Your build configuration (e.g. app/build.gradle and flavor-specific settings) loads the corresponding google-services.json file.

3. Apply the Google Services Plugin Correctly

It’s important to have the following line at the very bottom of your app-level build.gradle file:

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apply plugin: 'com.google.gms.google-services'

Double-check that this line hasn’t been accidentally moved or removed. It must be applied after the dependencies section.

4. Clean and Rebuild

Sometimes stale build artifacts can cause inconsistent behavior between debug and release. Run:

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flutter clean
flutter build apk --flavor prod --release

5. Check ProGuard/Minification (if enabled)

If you’re using minification or ProGuard in release builds, ensure you have the necessary rules so that Firebase classes aren’t stripped. Consult the Firebase ProGuard setup if needed.

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By confirming these settings, you should be able to resolve the "[DEFAULT]" app not created error in your release builds.