Object reference not set to an instance of an object - Unity Quiz [duplicate]

This question already has an answer here:

• What is a NullReferenceException, and how do I fix it? 23 answers

I am trying to make a Unity quiz game when u get random questions but it's not working it's showing me "Object reference not set to an instance of an object" and pointing to line 24, where i declare the array of questions.

QuestionGenerator class:

    using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class QuestionGenerator : ScriptableObject {

    private Question[] questions = new Question[3];
    public Text question;
    public Text[] answer;
    private int selection;

    void Start(){

        questions [0] = new Question ();
        questions [0].question = "Gannymede is the largest moon of which planet?";
        questions [0].correctAnswer = "Jupiter";
        questions [0].answers [0] = "Mars";
        questions [0].answers [1] = "Uranus";
        questions [0].answers [2] = "Jupiter";
        questions [0].answers [3] = "Neptune";


        questions [1] = new Question ();
        questions [1].question = "How many planets make up our Solar System?";
        questions [1].correctAnswer = "8";
        questions [1].answers [0] = "7";
        questions [1].answers [1] = "9";
        questions [1].answers [2] = "6";
        questions [1].answers [3] = "8";

        questions [2] = new Question ();
        questions [2].question = "One of the following is NOT an Overwatch hero";
        questions [2].correctAnswer = "Dokken";
        questions [2].answers [0] = "Ana";
        questions [2].answers [1] = "Tracer";
        questions [2].answers [2] = "Dokken";
        questions [2].answers [3] = "Pharah";



        selection = Random.Range (0,2);
        question.text  =  questions[selection].question; 



    }

    // Update is called once per frame
    void Update () {










    }






}

My question class (The base question class Question.cs):

    using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Question : MonoBehaviour {

    public string correctAnswer;
    public string[] answers;
    public string question;
    public string subject;


}




I've tried everything, retyping the whole deal and also cleaning, and nothing is working. please help! thank you very much.