Появилась следующая ошибка при разработке приложения в Android Studio: попытка вызвать виртуальный метод

Ошибка выглядит так "Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference"

private List<Questions> questionList;
    private int questionCounter;
    private int questionTotalCount;
    private Questions currentQuestions;
    private ColorStateList testColorDefaultRb;
    private  boolean answerd;
    private  int score;

    private Handler handler = new Handler();





    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_quiz);
        setupUI();
        fetchDB();

    }
    private  void setupUI(){
        textViewCorrect = findViewById(R.id.txtCorrect);
        textViewWrong = findViewById(R.id.txtWrong);
        textViewCountDown =  findViewById(R.id.txtViewTimer);
        textViewQuestionCount = findViewById(R.id.txtTotalQuestion);
        textViewScore = findViewById(R.id.txtScore);

         buttonConfirmNext=findViewById(R.id.button);
         rbGroup = findViewById(R.id.radio_group);
         rb1 = findViewById(R.id.radio_button1);
         rb2 = findViewById(R.id.radio_button2);
         rb3 = findViewById(R.id.radio_button3);
         rb4 = findViewById(R.id.radio_button4);

        testColorDefaultRb = rb1.getTextColors();




    }
    private void fetchDB(){
        QuizDbHelper dbHelper = new QuizDbHelper(this);
        questionList = dbHelper.getAllQuestions();
        startQuiz();
    }

    private void startQuiz() {
        questionTotalCount = questionList.size();
        Collections.shuffle(questionList);

        showQuestions();

        buttonConfirmNext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(!answerd){
                    if(rb1.isChecked() || rb2.isChecked() || rb3.isChecked() || rb4.isChecked()){

                        quizOperations();
                    }
                    else{
                        Toast.makeText(QuizActivity.this, "Выберите опцию", Toast.LENGTH_SHORT).show();
                    }
                }
            }
        });
    }

    private void quizOperations() {
        answerd = true;
        RadioButton rbselected = findViewById(rbGroup.getCheckedRadioButtonId());
        int answerNr = rbGroup.indexOfChild(rbselected)+1;
        checkSolution(answerNr, rbselected);
    }

    private void checkSolution(int answerNr, RadioButton rbselected) {
        switch (currentQuestions.getAnswerNr()){
            case 1:
                if(currentQuestions.getAnswerNr()==answerNr){
                    rb1.setBackground(ContextCompat.getDrawable(this, R.drawable.when_answers_correct));
                    showQuestions();
                }
                else {
                    changetoIncorrectColor(rbselected);
                    showQuestions();
                }

            case 2:
                if(currentQuestions.getAnswerNr()==answerNr){
                    rb2.setBackground(ContextCompat.getDrawable(this, R.drawable.when_answers_correct));
                    showQuestions();
                }
                else {
                    changetoIncorrectColor(rbselected);
                    showQuestions();
                }
                break;
            case 3:
                if(currentQuestions.getAnswerNr()==answerNr){
                    rb3.setBackground(ContextCompat.getDrawable(this, R.drawable.when_answers_correct));
                    showQuestions();
                }
                else {
                    changetoIncorrectColor(rbselected);
                    showQuestions();
                }
                break;
            case 4:
                if(currentQuestions.getAnswerNr()==answerNr){
                    rb4.setBackground(ContextCompat.getDrawable(this, R.drawable.when_answers_correct));
                    showQuestions();
                }
                else {
                    changetoIncorrectColor(rbselected);
                    showQuestions();
                }
                break;
        }
        if (questionCounter<questionTotalCount){
            buttonConfirmNext.setText("Подтвердить и закончить");
        }
    }


    private void changetoIncorrectColor(RadioButton rbselected) {
        rbselected.setBackground(ContextCompat.getDrawable(this, R.drawable.when_answers_wrong));
    }

    public void showQuestions(){
    rbGroup.clearCheck();
    if(questionCounter<questionTotalCount){
        currentQuestions = questionList.get(questionCounter);
        textViewQuestions.setText(currentQuestions.getQuestion());
        rb1.setText(currentQuestions.getOption1());
        rb2.setText(currentQuestions.getOption2());
        rb3.setText(currentQuestions.getOption3());
        rb4.setText(currentQuestions.getOption4());

        questionCounter++;
        answerd = false;

        buttonConfirmNext.setText("Подтверждаю");
        textViewQuestionCount.setText("Вопрос :" +questionCounter + "/" + questionTotalCount);

    }
    else{
        handler.postDelayed(new Runnable() {
            @Override
            public void run() {
                Intent intent = new Intent(getApplicationContext(),QuizActivity.class);
                startActivity(intent);

            }
        },500);
    }
    }
}