setTextを使うとnullpointerexceptionとエラーが出ることについて

前提・実現したいこと

NullPointerExceptionエラーが出て困ってます><

発生している問題・エラーメッセージ

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
        at com.example.quizdb.QuizActivity.showNextQuestion(QuizActivity.java:88)

該当のソースコード

package com.example.quizdb;

import androidx.appcompat.app.AppCompatActivity;

import android.content.res.ColorStateList;
import android.graphics.Color;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;

import java.util.Collections;
import java.util.List;

public class QuizActivity extends AppCompatActivity {
private TextView textViewQuestion;
private TextView textViewScore;
private TextView textViewQuestionCount;
private TextView textViewCountDown;
private RadioGroup rbGroup;
private RadioButton rb1;
private RadioButton rb2;
private RadioButton rb3;
private Button buttonConfirmNext;

private ColorStateList textColorDefaultRb;

private List<Question> questionList;
private int questionCounter;
private int questionCountTotal;
private Question currentQuestion;

private int score;
private boolean answered;

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

textViewQuestion = findViewById(R.id.text_view_question);
textViewScore = findViewById(R.id.text_view_score);
textViewQuestionCount = findViewById(R.id.text_view_question_count);
textViewCountDown = findViewById(R.id.text_view_countdown);
rbGroup = findViewById(R.id.radio_group);
rb1 = findViewById(R.id.radio_button1);
rb2 = findViewById(R.id.radio_button2);
rb3 = findViewById(R.id.radio_button3);
buttonConfirmNext = findViewById(R.id.button_confirm_next);

textColorDefaultRb = rb1.getTextColors();

QuizDbHelper dbHelper = new QuizDbHelper(this);
questionList = dbHelper.getAllQuestions();
questionCountTotal = questionList.size();
Collections.shuffle(questionList);

showNextQuestion();

buttonConfirmNext.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (!answered) {
if (rb1.isChecked() || rb2.isChecked() || rb3.isChecked()) {
checkAnswer();
} else {
Toast.makeText(QuizActivity.this, "Please select an answer", Toast.LENGTH_SHORT).show();
}
} else {
showNextQuestion();
}
}
});
}

private void showNextQuestion() {
rb1.setTextColor(textColorDefaultRb);
rb2.setTextColor(textColorDefaultRb);
rb3.setTextColor(textColorDefaultRb);
rbGroup.clearCheck();

if (questionCounter < questionCountTotal) {
currentQuestion = questionList.get(questionCounter);

textViewQuestion.setText(currentQuestion.getQuestion());
rb1.setText(currentQuestion.getOption1());
rb2.setText(currentQuestion.getOption2());
rb3.setText(currentQuestion.getOption3());

questionCounter++;
textViewQuestionCount.setText("Question: " + questionCounter + "/" + questionCountTotal);
answered = false;
buttonConfirmNext.setText("Confirm");
} else {
finishQuiz();
}
}

private void checkAnswer() {
answered = true;

RadioButton rbSelected = findViewById(rbGroup.getCheckedRadioButtonId());
int answerNr = rbGroup.indexOfChild(rbSelected) + 1;

if (answerNr == currentQuestion.getAnswerNr()) {
score++;
textViewScore.setText("Score: " + score);
}

showSolution();
}

private void showSolution() {
rb1.setTextColor(Color.RED);
rb2.setTextColor(Color.RED);
rb3.setTextColor(Color.RED);

switch (currentQuestion.getAnswerNr()) {
case 1:
rb1.setTextColor(Color.GREEN);
textViewQuestion.setText("Answer 1 is correct");
break;
case 2:
rb2.setTextColor(Color.GREEN);
textViewQuestion.setText("Answer 2 is correct");
break;
case 3:
rb3.setTextColor(Color.GREEN);
textViewQuestion.setText("Answer 3 is correct");
break;
}

if (questionCounter < questionCountTotal) {
buttonConfirmNext.setText("Next");
} else {
buttonConfirmNext.setText("Finish");
}
}

private void finishQuiz() {
finish();
}
}