NoSuchMethodError：在null上呼叫了getter'isNotEmpty'

我試圖在可用時將當前答案作為提示文本。但我收到了這個錯誤：NoSuchMethodError: The getter 'isNotEmpty' was called on null.
關于如何解決它的任何想法？

  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",
                    ),
                  ),
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }
}

我應該使用一些狀態管理還是有其他方法可以做到這一點？{這是一個隨機文本，所以 stackoverflow 接受我的問題，因為它說我的問題主要是代碼，我需要添加更多細節}

uj5u.com熱心網友回復：

在 Dart 中，當從 map 中訪問值時，回傳的值可以是 Null 或存盤在 map 中的實際值。在您的情況下，在您嘗試訪問 的那一刻answers[question]，沒有值 - 回傳 null 。然后，您嘗試呼叫isNotEmptyon null，您會收到錯誤訊息。

要解決此問題，請替換此：

decoration: InputDecoration(
  hintText: answers[question].isNotEmpty
    ? answers[question]
    : "write something",
),

有了這個：

decoration: InputDecoration(
  hintText: answers[question]?.isNotEmpty ?? false
    ? answers[question]
    : "write something",
),

在此示例中，如果answers[question]為 null，則該陳述句將屬于 false 條件，您的代碼不會中斷。

有關更多資訊，請檢查Dart 中的 null 安全性。

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