Flutter中的单例给出运行时错误“未处理的异常:在初始化期间读取静态变量'_instance@545324594‘”

class Singleton {
  static Singleton? _instance;

  Singleton._internal();

  factory Singleton() {
    if (_instance == null) {
      _instance = Singleton._internal();
    }
    return _instance!;
  }
}

import 'package:synchronized/synchronized.dart';

class Singleton {
  static Singleton? _instance;
  static final _lock = Lock();

  Singleton._internal();

  factory Singleton() {
    _lock.synchronized(() {
      if (_instance == null) {
        _instance = Singleton._internal();
      }
    });
    return _instance!;
  }
}

class Singleton {
  static late Singleton _instance;

  Singleton._internal();

  factory Singleton() {
    if (_instance == null) {
      _instance = Singleton._internal();
    }
    return _instance;
  }
}

class Singleton {
  static late Singleton _instance;

  Singleton._internal();

  factory Singleton() {
    if (_instance == null) {
      _instance = Singleton._internal();
    }
    return _instance;
  }

  void doSomething() {
    print("Singleton instance is doing something.");
  }
}

void main() {
  Singleton().doSomething();
  Singleton().doSomething(); // 确保是同一个实例
}