NoSuchMethodError: The getter 'isNotEmpty' was called on null

Question

im trying to put the current answer as a hint text when it's available. but i'm getting this error: NoSuchMethodError: The getter 'isNotEmpty' was called on null.
any ideas on how to fix it?

  final Map<String, String> answers = {};
  final Map<String, String> qst = {
    "Everyone should read...": "",
    "Two truths and a lie...": "",
    "I can quote every line from...": "",
    "If I didn't have to work I would...": "",
    "People think I am...": "",
    "Never have I ever...": "",
    "Believe it or not, I...": "",
    "I am amazing at...": "",
    "My life as a movie...": "",
    "My ultimate dinner party guest list...": "",
    "The dorkiest thing about me is...": "",
    "On the weekend you'll find me...": "",
  };
  Future<void> _write(Map<String, dynamic> map) async {
    final directory = await getApplicationDocumentsDirectory();

    final jsonStr = jsonEncode(map);

    final file = File('${directory.path}/answers.txt');

    await file.writeAsString(jsonStr);
  }

  List<Data> data = [];
  @override
  void initState() {
    super.initState();
    read();
  }

  Future<Map<String, dynamic>> read() async {
    final directory = await getApplicationDocumentsDirectory();

    final file = File('${directory.path}/answers.txt');
    final jsonStr = await file.readAsString();
    final raw = jsonDecode(jsonStr) as Map<String, dynamic>;
    raw.forEach((key, value) {
      data.add(Data(question: key, answer: value));
    });
    print(raw);
    return raw;
  }

  @override
  Widget build(
    BuildContext context,
  ) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        for (var q in qst.keys) question(q),
        SizedBox(
          height: 50,
        ),
      ],
    );
  }

  Widget question(String question) {
    final myController = TextEditingController();
    return Padding(
      padding: const EdgeInsets.only(top: 15),
      child: Stack(
        children: [
          Card(
            elevation: 0,
            child: Padding(
              padding: const EdgeInsets.all(20.0),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Padding(
                    padding: const EdgeInsets.only(bottom: 15, left: 10),
                    child: Text(
                      question,
                      style: TextStyle(
                          color: Colors.black,
                          fontSize: 18.0,
                          fontWeight: FontWeight.bold),
                    ),
                  ),
                  TextField(
                    onChanged: (String ansr) {
                      answers[question] = ansr;
                    },
                    onSubmitted: (String ansr) {
                      _write(answers);
                    },
                    controller: myController,
                    decoration: InputDecoration(
                      hintText: answers[question].isNotEmpty
                          ? answers[question]
                          : "write something",
                    ),
                  ),
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

should i use some state management or is there any way to do it another way? {this's a random text so stackoverflow accept my question because it says my question is mostly code and I need to add some more details}