A non-null String must be provided to a Text widget . error shows but null-safety doesnt make it for the code

i'm new to programming and was following an older course that has no null-safety . i tried to solve the problem by insertin a default value , but that just made all the texts on the buttons have the word "default value" on them .

    // @dart=2.9

import 'package:flutter/material.dart';

class Answer extends StatelessWidget {
  final Function selectHandler;
  final String answerText;

  Answer(this.selectHandler, this.answerText);

  @override
  Widget build(BuildContext context) {
    return Container(
        width: double.infinity,
        child: RaisedButton(
          color: Colors.red.shade400,
          textColor: Colors.white,

          child: Text(answerText?? 'default value'),

          onPressed: selectHandler,
        ));
  }
}

EDIT :

the Answer class gets called here :

// @dart=2.9

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

class Quiz extends StatelessWidget {
  final List<Map<String, Object>> questions;
  final int questionIndex;
  final Function answerQuestion;

  Quiz(
      {@required this.questions,
      @required this.answerQuestion,
      @required this.questionIndex});

  @override
  Widget build(BuildContext context) {
    return ListView(
      children: [
        Question(
         questions[questionIndex]['questionText']?? "default value",
        ),
        ...(questions[questionIndex]['answers'] as List<Map<String, Object>>)
            .map((answer) {
          return Answer(() => answerQuestion(answer['score']), answer['text']);
        }).toList()
      ],
    );
  }
}

Solution 1: ImPiNoron

In null-safety dart, a String will never be null so you can't use ?? after a string. However, you can define answerText as a String? which will make it a nullable string. For more details, you can check this website.

The above explanation can make the program run but setting default values everywhere is not a good coding style. It will cause a lot of troubles when you or your team is trying to debug your code. in A better practice would be setting default values when initiating the object:

class Answer extends StatelessWidget {
  final void Function()? selectHandler;
  final String answerText;

  Answer(void Function()? selectHandler, String? answerText)
      : selectHandler = selectHandler,
        answerText = answerText ?? 'default value';

  @override
  Widget build(BuildContext context) {
    return Container(
        width: double.infinity,
        child: RaisedButton(
          color: Colors.red.shade400,
          textColor: Colors.white,
          child: Text(answerText),
          onPressed: selectHandler,
        ));
  }
}