您的代码中有两个问题：

1. 在体内使用异步方法initState() 见这里了解详细信息

2. 在初始化程序中使用实例数据，请参见此处以获取详细信息

接下来的工作是对您的代码进行非常基本的重写，并进行了最少的更正。

数据映射从模拟的后端加载，在内部更新PageOne并在PageTwoonTap回调中打印到控制台。

请注意，我已经改变了实例变量Data，以data使其符合有效飞镖准则。

请注意，要点无法正确地解决具有共享首选项的后端服务的同步：这可能是最终产品中必须考虑的问题。

我只是评论了使代码正常工作的必要条件：如果系统的复杂性以及与外部API的关系开始增长，那么可能值得考虑使用Bloc架构。

import 'package:flutter/material.dart';

void main() => runApp(new MainApp());

// Mock up of an async backend service
Future<Map<String, dynamic>> getData() async {
  return Future.delayed(Duration(seconds: 1), () => {'prop1': 'value1'});
}

class PageOne extends StatelessWidget {
  final Map<String, dynamic> data;

  PageOne({Key key, this.data}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Center(
      child: RaisedButton(
        child: const Text('update preferences'),
        onPressed: () {
          data['prop2'] = 'value2';
        },
      ),
    );
  }
}

class PageTwo extends StatelessWidget {

  final Map<String, dynamic> data;

  PageTwo({Key key, this.data}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Center(
      child: RaisedButton(
        child: const Text('Got It!'),
        onPressed: () {
          print("data is now: [$data]");
        },
      ),
    );
  }
}

class MainApp extends StatefulWidget {
  @override
  _MainAppState createState() => _MainAppState();
}

class _MainAppState extends State<MainApp> {
  //Map<String, dynamic> Data;
  Map<String, dynamic> data;

  /*
  StartFunc() async {
    Data = await getData();
    setState(() {});
  }
  */

  @override
  void initState() {
    //StartFunc();
    super.initState();
    getData().then((values) {
      setState(() {
        data = values;
      });
    });
  }

  /*
  PageOne(data:data) is an invalid value for an initializer:
   there is no way to access this at this point.
    Initializers are executed before the constructor,
    but this is only allowed to be accessed after the call
    to the super constructor.

  */
  /*
  var _pages = [
    PageOne(data:data),
    PageTwo(),
  ];
  */

  Widget getPage(int index) {
    if (index == 0) {
      return PageOne(data:data);
    }
    if (index == 1) {
      return PageTwo(data:data);
    }
    // A fallback, in this case just PageOne
    return PageOne();
  }

  int _currentIndex = 0;

  onTabTapped(int index) {
    setState(() {
      _currentIndex = index;
    });
  }

  @override
  Widget build(BuildContext context) {
    /*
    return _currentIndex == 2
        ? PageTwo()
        : Scaffold(

    I use a MaterialApp because of material widgets (RaisedButton)
    It is not mandatory, but it is mainstream in flutter

     */
    return MaterialApp(
        title: 'My App',
        home: Scaffold(
          appBar: AppBar(title: Text("My App Bar")),
          body: getPage(_currentIndex),
          bottomNavigationBar: BottomNavigationBar(
            type: BottomNavigationBarType.fixed,
            items: [
              BottomNavigationBarItem(
                  icon: Icon(Icons.first_page), title: Text('')),
              BottomNavigationBarItem(
                  icon: Icon(Icons.last_page), title: Text('')),
            ],
            onTap: onTabTapped,
            currentIndex: _currentIndex,
          ),
        ));
  }
}