Flutter – _TypeError (type 'Null' is not a subtype of type '() => void' of 'function result')

I am getting this error while debugging my flutter app , there is no error in my dart files but during debug I get this error . I am attaching my dart files here .

main.dart

import 'package:flutter/material.dart';

import 'package:quiz_app2/quiz.dart';

void main() {
  runApp(const Quiz());
}

quiz.dart

import 'package:flutter/material.dart';

import 'package:quiz_app2/firstscreen.dart';
import 'package:quiz_app2/questions.dart';

class Quiz extends StatefulWidget {
  const Quiz({key}) : super(key: key);

  @override
  State<Quiz> createState() {
    return _QuizState();
  }
}

class _QuizState extends State<Quiz> {
  Widget? activeScreen;

  @override
  void initState() {
    activeScreen = FirstScreen(switchScreen);
    super.initState();
  }

  void switchScreen() {
    setState(() {
      activeScreen = const QuestionScreen();
    });
  }

  @override
  Widget build(context) {
    return MaterialApp(
      home: Scaffold(
        body: Container(
          decoration: const BoxDecoration(
            gradient: LinearGradient(
              colors: [Colors.deepPurple, Colors.deepPurpleAccent],
            ),
          ),
          child: activeScreen,
        ),
      ),
    );
  }
}

firstScreen.dart <– The error is showing at this file while debug .

import 'package:flutter/material.dart';

class FirstScreen extends StatelessWidget {
  const FirstScreen(this.startQuiz, {key}) : super(key: key);

  final void Function() startQuiz;
  @override
  Widget build(context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          Image.asset(
            'assets/images/quiz-logo.png',
            height: 400,
            color: const Color.fromARGB(150, 250, 250, 250),
          ),
          const Padding(
            padding: EdgeInsets.only(top: 40),
            child: Text(
              'Learn Flutter the fun way!',
              style: TextStyle(
                color: Color.fromARGB(255, 255, 255, 255),
                fontSize: 30,
              ),
            ),
          ),
          const SizedBox(height: 40),
          OutlinedButton.icon(
            onPressed: startQuiz,
            style: OutlinedButton.styleFrom(
              foregroundColor: Colors.white,
            ),
            icon: const Icon(Icons.arrow_right_alt),
            label: const Text(
              'Start Quiz',
              style: TextStyle(fontSize: 20),
            ),
          ),
        ],
      ),
    );
  }
}

questions.dart

import 'package:flutter/material.dart';

class QuestionScreen extends StatefulWidget {
  const QuestionScreen({key}) : super(key: key);

  @override
  State<QuestionScreen> createState() {
    return _QuestionScreenState();
  }
}

class _QuestionScreenState extends State<QuestionScreen> {
  @override
  Widget build(context) {
    return const Text('questions');
  }
}

I want the solution of this error , I have searched but not got the answer. Thanking you in anticipation.