方法格式的完整日期在null上被调用

Question

我试图在波斯日期中使用flutter datepicker，但是当我在日期选择器上单击时，出现此错误：

  

方法格式的完整日期被调用为空

我很陌生，我不确定是否使用了正确的语言环境和日期时间。 这是我的代码：

class ProfileSettingPage extends StatefulWidget {
  @override
  _ProfileSettingPageState createState() => _ProfileSettingPageState();
}

class _ProfileSettingPageState extends State<ProfileSettingPage> {
  final TextEditingController _birthDayController = new TextEditingController();
  DateTime selectedDate = DateTime.now();

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        body: _buildProfileSetting(context),
      ),
    );
  }

  _buildProfileSetting(BuildContext context) {
    return Form(
        child: ListView(
      children: <Widget>[
        Directionality(
          textDirection: TextDirection.rtl,
          child: TextField(
            textInputAction: TextInputAction.go,
            onChanged: (value) {},
            onTap: () => _selectDate(context),
            controller: _birthDayController,
            decoration: InputDecoration(
              enabled: true,
              labelText: "birth day",
          ),
        ),
      ],
    ));
  }

  Future<Null> _selectDate(BuildContext context) async {
    final DateTime picked = await showDatePicker(
        context: context,
        locale: Locale("fa"),
        initialDate: selectedDate,
        firstDate: DateTime(2015, 8),
        lastDate: DateTime(2101));
    if (picked != null && picked != selectedDate)
      _birthDayController.text = picked.toIso8601String();
      print(picked);
  }
}

有什么想法吗？