[Solved] LateInitializationError: Field has not been initialized Error in Flutter

The LateInitializationError in Flutter occurs when you declare a variable with the late keyword but fail to assign it a value before it’s used for the first time.

In simple terms, you’ve made a promise to Dart that a variable will have a value before you read from it, but that promise was broken.

Common Causes and Solutions

This error most often appears in a few common scenarios, especially within StatefulWidget classes.

1. Forgetting to Initialize in initState

A frequent mistake is declaring a late variable in a StatefulWidget‘s State class and then forgetting to initialize it inside the initState method.

Problem:

class _MyWidgetState extends State<MyWidget> {
  late AnimationController _controller;

  @override
  Widget build(BuildContext context) {
    // ERROR! _controller is used here before it's ever initialized.
    return Transform.rotate(
      angle: _controller.value,
      child: const FlutterLogo(size: 100),
    );
  }
  
  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }
}

Solution:

Initialize the variable in initState(), which is guaranteed to run before build().

class _MyWidgetState extends State<MyWidget> {
  late AnimationController _controller;

  @override
  void initState() {
    super.initState();
    // Initialize the variable here
    _controller = AnimationController(
      vsync: this, // requires with TickerProviderStateMixin
      duration: const Duration(seconds: 2),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Transform.rotate(
      angle: _controller.value,
      child: const FlutterLogo(size: 100),
    );
  }
  
  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }
}

class _MyWidgetState extends State<MyWidget> {
  late AnimationController _controller;

  @override
  void initState() {
    super.initState();
    // Initialize the variable here
    _controller = AnimationController(
      vsync: this, // requires with TickerProviderStateMixin
      duration: const Duration(seconds: 2),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Transform.rotate(
      angle: _controller.value,
      child: const FlutterLogo(size: 100),
    );
  }
  
  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }
}

2. Variable Initialized Conditionally

If the logic that initializes your late variable is inside a condition that doesn’t run, the variable will remain uninitialized.

Problem:

class User {
  late String name;

  void loadName(bool hasNetwork) {
    if (hasNetwork) {
      this.name = 'Flutter Dev'; // This line might not run!
    }
  }
}

void main() {
  final user = User();
  user.loadName(false); // We pass false
  print(user.name); // ERROR! 'name' was never initialized.
}

class User {
  late String name;

  void loadName(bool hasNetwork) {
    if (hasNetwork) {
      this.name = 'Flutter Dev'; // This line might not run!
    }
  }
}

void main() {
  final user = User();
  user.loadName(false); // We pass false
  print(user.name); // ERROR! 'name' was never initialized.
}

Solution:

Ensure all code paths initialize the variable, or consider making the variable nullable (?) if its existence is truly optional.

class User {
  String? name; // Use a nullable type instead

  void loadName(bool hasNetwork) {
    if (hasNetwork) {
      this.name = 'Flutter Dev';
    }
  }
}

void main() {
  final user = User();
  user.loadName(false);
  print(user.name); // Prints 'null' - no error
}

class User {
  String? name; // Use a nullable type instead

  void loadName(bool hasNetwork) {
    if (hasNetwork) {
      this.name = 'Flutter Dev';
    }
  }
}

void main() {
  final user = User();
  user.loadName(false);
  print(user.name); // Prints 'null' - no error
}

3. Using late final without Immediate Initialization

A late final variable must be assigned a value exactly once. If you declare it without an immediate value, you must initialize it later (e.g., in a constructor or initState) before it’s used.

Problem:

class ApiConfig {
  late final String apiKey;

  ApiConfig() {
    // Constructor finishes without initializing apiKey
  }

  void connect() {
    print('Connecting with key: $apiKey'); // ERROR! apiKey has no value.
  }
}

class ApiConfig {
  late final String apiKey;

  ApiConfig() {
    // Constructor finishes without initializing apiKey
  }

  void connect() {
    print('Connecting with key: $apiKey'); // ERROR! apiKey has no value.
  }
}

Solution:

Initialize the late final variable in the constructor or from another source before it’s accessed.

class ApiConfig {
  late final String apiKey;

  // Initialize via the constructor
  ApiConfig(this.apiKey);

  void connect() {
    print('Connecting with key: $apiKey');
  }
}

class ApiConfig {
  late final String apiKey;

  // Initialize via the constructor
  ApiConfig(this.apiKey);

  void connect() {
    print('Connecting with key: $apiKey');
  }
}

How to Prevent This Error

• Initialize in initState: For state-dependent data in a StatefulWidget, initState is the perfect place for initialization.
• Use Nullable Types (?): If a variable is allowed to be null, declare it with a ? (e.g., String? name;) instead of using late. This is often a safer alternative.
• Initialize in Constructor: If a variable is essential for a class to function, require it in the constructor.
• Check Your Logic: Before using a late variable, double-check that all possible code paths leading to it will assign it a value.