Исключение NullPointer, предложения? [Дубликат]

Я следую за большой книгой Big Java Late Objects, и у меня есть проблема с решением задач. Речь идет о наследовании. Существует класс Question и один дополнительный класс ChoiceQuestion, который расширяет его.

Но когда я запускаю код в своем тестовом классе, он вызывает нулевое исключение, и я не знаю, почему.

Exception in thread "main" java.lang.NullPointerException
at ChoiceQuestion.addChoice(ChoiceQuestion.java:12)
at TestClass.main(TestClass.java:21)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at                  sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Вот мои классы:

public class Question {
private String text;
private String answer;

/**
 *  Constructs a queston with empty question and answer.
 */
public Question(){
    text = "";
    answer = "";
}
//=============Setter methods============
public void setText(String questionText){
    text = questionText;
}

public void setAnswer(String correctResponse){
    answer = correctResponse;
}
//==============Getter Methods=============
public boolean checkAnswer(String response){
    return answer.equals(response);
}
public void display(){
    System.out.println(text);
}
}

И вот мой подкласс:

class ChoiceQuestion extends Question {
private ArrayList<String> choices;


//=========Methods==============
public void addChoice(String choice,boolean correct){
    choices.add(choice);
    if(correct){
        //convert choices.size() to string
        String choiceString = "" + choices.size();
       this.setAnswer(choiceString);
    }
}

public void display(){
    // Display the question text
    super.display();
    // Display the answer choices
    for(int i=0;i<choices.size();i++){
        int choiceNumber = i + 1;
        System.out.println(choiceNumber + ": " + choices.get(i));
    }
}
}

И мой тест Класс:

import java.util.Scanner;

public class TestClass {
public static void presentQuestion(Question q){
    q.display();
    System.out.println("Your answer: ");
    Scanner in = new Scanner(System.in);
    String response = in.nextLine();
    System.out.println(q.checkAnswer(response));
}
public static void main(String[] args) {
    Question first = new Question();
    first.setText("James Gosling");
    first.setAnswer("James Gosling");

    ChoiceQuestion second = new ChoiceQuestion();
    second.setText("In which country was the inventor of Java born? ");
    second.addChoice("Austria", false);
    second.addChoice("Canada",true);
    second.addChoice("Denmark",false);
    second.addChoice("United States",false); 
    presentQuestion(first);
    presentQuestion(second);

}
}