i在这里
中找到了解决方案 Flutter：启动时共享首选项为空
，请注意，如果您像在
之前那样在主类之外调用它，则
共享首选项将不起作用，您应该在initState（）
内部调用它

i found the solution in here Flutter: Shared Preferences null on Startup and note that shared preference will not work if you called it outside the main class like i did before you should call it inside initState() this works

@override
  void initState() {
    super.initState();
    readFile('phone');
    SharedPreferences.getInstance().then((SharedPreferences sp) {
      sharedPreferences = sp;
      ph = sp.getString('phone');
      setState(() {});
    });
  }