Dart: type 'Null' is not a subtype of type 'String' in type cast

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

// Fails the moment the key is absent or null
final name = json['name'] as String;

// Say that it may be missing, and decide what that means
final name = json['name'] as String?;
final name = json['name'] as String? ?? '';

// Numbers need care as well: JSON integers decode as int, not double
final price = (json['price'] as num).toDouble();

// Put the handling in one place rather than at every call site
factory User.fromJson(Map<String, dynamic> json) => User(
      name: json['name'] as String? ?? 'unknown',
      age: json['age'] as int?,
    );

# Generated models catch the mismatch at build time instead
dart run build_runner build --delete-conflicting-outputs

How to diagnose Dart errors

Dart errors are dominated by null safety at runtime and version solving at build time. The null-check operator failure (! on a null) is Dart's equivalent of a NullPointerException and almost always means an assumption about initialisation order was wrong. Pub version solving failures are dependency-graph conflicts and are solved by reading the constraint that pub reports as unsatisfiable, not by deleting the lockfile.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Read the full pub get output. It names the two packages whose constraints conflict. Deleting pubspec.lock hides that information without fixing anything.
2. Replace ! with a null check plus a meaningful error, or with late final where initialisation genuinely happens before first use. The crash location is where the assumption broke.
3. Run flutter doctor -v before debugging any build failure. It catches missing Android SDK components and licence acceptance, which produce misleading Gradle errors.
4. Use dart pub deps --style=tree to see the resolved graph and find which transitive dependency is pinning an old version.
5. Clear derived state with flutter clean only after you have read the error. It resets the symptom and loses the evidence.

Tools worth reaching for

• flutter doctor -v
• dart pub deps
• dart analyze
• flutter run --verbose

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 dart.dev 📖 dart.dev

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