i am trying to use map function in flutter

VSCode show me this problem notification:
Exception has occurred.

NoSuchMethodError (NoSuchMethodError: The method ‘map’ was called on
null. Receiver: null Tried calling: map(Closure: (String) =>
Answer))

this is the main library code:

import 'package:flutter/material.dart';
import './question.dart';
import 'answer.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return MyAppState();
  }
}

class MyAppState extends State<MyApp> {
  var questionIndex = 0;
  void answerquestion() {
    setState(() {
      if (questionIndex < 1) {
        questionIndex = questionIndex + 1;
      }
    });

    print("answer question!");
  }

  @override

  Widget build(BuildContext context) {
    var questions = [
      {
        "questionText": "where are you from?",
        "answers": ["berlin ", "london", "DC", "new york"]
      },
      {
        "questionText": "what is your favourite color?",
        "answers": [" purple", "red", "yellow ", "blue"]
      }
    ];
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text("jini question"),
        ),
        body: Column(children: [
          Question(
            questions[questionIndex]["questionText"],
          ),
          ...(questions[questionIndex]["answer"] as List<String>).map((answer) {
            return Answer(answerquestion, answer);
          }).toList()
        ]),
      ),
    );
  }
}

i am trying to use map function but i got this error message