Flutter FutureBuilder始终返回null

FutureBuilder<T>(
  future: Future<T>, // 异步操作的Future
  builder: (BuildContext context, AsyncSnapshot<T> snapshot) {
    // 根据异步操作的状态构建UI
  },
)

Future<String> fetchData() async {
  await Future.delayed(Duration(seconds: 2));
  return 'Hello, World!';
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder<String>(
      future: fetchData(),
      builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
        if (snapshot.connectionState == ConnectionState.waiting) {
          return CircularProgressIndicator(); // 显示加载指示器
        } else if (snapshot.hasError) {
          return Text('Error: ${snapshot.error}'); // 显示错误信息
        } else {
          return Text('Data: ${snapshot.data}'); // 显示返回的数据
        }
      },
    );
  }
}