Dart Flutter 在 main 中读取共享首选项，然后决定哪个启动页面

我想判断在主界面中启动哪个页面(实际上是登录页面和主页).所以我必须在首选项中阅读isLogin.主要怎么做？

我把这些代码Bundle 在一起:

Future<Null> checkIsLogin() async {
  String _token = "";
  // If token exist already, then HomePage
  SharedPreferences prefs = await SharedPreferences.getInstance();
  _token = prefs.getString("token");
  print('get token from prefs: ' +  _token);
  if (_token != "" && _token != null) {
    // already login
    print("alreay login.");
    isLogin = true;
  }
}

void main() {
  App.init();
  // if we have token then go to HomePage directly otherwise go to LoginPage.
  Widget _defaultHome = new LoginPage();
  checkIsLogin();
  if (isLogin) {
    _defaultHome = new HomePage();
  }

  runApp(new MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: globalThemeData,
      home: _defaultHome
  ));
}

在上面的代码中，isLogin是一个全局变量.出现错误:

Performing full restart...                                       
Restarted app in 2,810ms.
[VERBOSE-2:dart_error.cc(16)] Unhandled exception:
Invalid argument(s)
#0      _StringBase.+ (dart:core/runtime/libstring_patch.dart:245:57)
#1      checkIsLogin (file:///Volumes/xs/awesome/uranus/clients/flutter/flutter_asgard/lib/main.dart:17:34)
<asynchronous suspension>
#2      main (file:///Volumes/xs/awesome/uranus/clients/flutter/flutter_asgard/lib/main.dart:29:3)
#3      _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:279:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:165:12)

似乎主要有调用异步的问题，如何让它工作？