Null Safety Dependencies Error in Flutter

When migrating your Flutter application to null safety, you might encounter the error: "Cannot run with sound null safety, because the following dependencies don't support null safety." This occurs when your app uses packages that haven't yet been migrated to support Dart's null safety feature.

Problem: Mixed Null Safety Environments

Flutter's sound null safety prevents apps from running when dependencies aren't fully migrated. Common packages that previously caused this issue included:

• cloud_firestore_web
• firebase_core_web
• shared_preferences
• url_launcher_web
• firebase_auth
• http
• provider

Note

Most of these packages now support null safety, but you may still encounter similar issues with other dependencies.

Solutions

1. Update Dependencies (Recommended)

Before using workarounds, check if updated null-safe versions of your dependencies exist:

# pubspec.yaml
dependencies:
  # Check for null-safe versions:
  shared_preferences: ^2.0.0
  http: ^0.13.0
  provider: ^6.0.0

Check for null-safe packages using:

dart pub outdated --mode=null-safety

2. Run with Unsound Null Safety (Temporary Fix)

To run your app with mixed null safety:

flutter run --no-sound-null-safety

For specific build targets:

# For APK
flutter build apk --no-sound-null-safety

# For App Bundle
flutter build appbundle --no-sound-null-safety

# For iOS
flutter build ipa --no-sound-null-safety

# For specific devices
flutter run -d chrome --no-sound-null-safety

3. IDE Configuration

VS Code - Settings.json

{
  "dart.flutterRunAdditionalArgs": ["--no-sound-null-safety"],
  "dart.flutterTestAdditionalArgs": ["--no-sound-null-safety"]
}

VS Code - launch.json

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Flutter",
      "request": "launch",
      "type": "dart",
      "args": ["--no-sound-null-safety"]
    }
  ]
}

Android Studio/IntelliJ:

1. Open Run → Edit Configurations
2. Add --no-sound-null-safety to "Additional run args"

4. File-level Opt-Out (Legacy Approach)

Add this directive at the top of your main.dart file:

// @dart=2.9
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

Warning

This approach uses legacy Dart 2.9 behavior and is not recommended for new projects. Use temporary runtime flags instead.

Best Practices

1. Prioritize dependency updates over workarounds
2. Check package status at pub.dev before migration
3. Test thoroughly after enabling sound null safety
4. Remove --no-sound-null-safety flags once all dependencies support null safety

Migration Checklist

• [ ] Verify all dependencies have null-safe versions
• [ ] Run dart pub upgrade --null-safety
• [ ] Test with --no-sound-null-safety temporarily
• [ ] Gradually migrate remaining non-null-safe code
• [ ] Remove null safety flags once fully migrated

Tip

Most popular packages now support null safety. If you encounter this error, first check for package updates rather than immediately disabling sound null safety.

For more information, see Dart's official guide on unsound null safety.