How to fix messaging/invalid-package-name: Package name doesn't match in Firebase

// android/app/build.gradle
android {
    defaultConfig {
        applicationId "com.example.myapp"  // This is your package name
        // ...
    }
}

<!-- android/app/src/main/AndroidManifest.xml -->
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapp">
    <!-- ... -->
</manifest>

{
  "project_info": { ... },
  "client": [
    {
      "client_info": {
        "mobilesdk_app_id": "1:123456789:android:abc123...",
        "android_client_info": {
          "package_name": "com.example.myapp"
        }
      }
    }
  ]
}

// android/app/build.gradle
android {
    buildTypes {
        debug {
            applicationIdSuffix ".debug"
            // This creates package: com.example.myapp.debug
        }
        release {
            // This uses: com.example.myapp
        }
    }
}

android {
    buildTypes {
        debug {
            applicationIdSuffix ".debug"
        }
    }
}

// Copy the appropriate google-services.json based on build type
task switchToDebug {
    doLast {
        copy {
            from 'google-services-debug.json'
            into 'app'
            rename { 'google-services.json' }
        }
    }
}

# Clean Gradle cache
cd android
./gradlew clean

# Clear build folders
rm -rf app/build
rm -rf build

# For React Native projects
cd ..
rm -rf node_modules
npm install

# For Flutter projects
flutter clean
flutter pub get

# Android
./gradlew assembleDebug

# React Native
npx react-native run-android

# Flutter
flutter run

// Android (Kotlin)
import com.google.firebase.Firebase
import com.google.firebase.messaging.FirebaseMessaging

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // Test FCM token retrieval
        FirebaseMessaging.getInstance().token.addOnCompleteListener { task ->
            if (task.isSuccessful) {
                val token = task.result
                Log.d("FCM", "Token retrieved successfully: $token")
            } else {
                Log.e("FCM", "Failed to get token", task.exception)
            }
        }
    }
}