Challenge: Closing Loopholes for Cheater

Here is my code:

MainActivity.kt

private const val TAG = "MainActivity"
private const val KEY_INDEX = "index"
private const val  REQUEST_CODE_CHEAT = 0
private const val EXTRA_ANSWER_SHOWN = "answer_is_shown"
private const val IS_CHEATER = "isCheater"
class MainActivity : AppCompatActivity() {
    private lateinit var trueButton: Button
    private lateinit var falseButton: Button
    private lateinit var nextButton: Button
    private lateinit var cheatButton: Button
    private lateinit var questionTextView: TextView

    private val quizModel: QuizViewModel by lazy {
        ViewModelProviders.of(this).get(QuizViewModel::class.java)
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        Log.i(TAG,"onSaveInstanceState")
        outState.putInt(KEY_INDEX,quizModel.currentIndex)
        outState.putBoolean(IS_CHEATER,quizModel.isCheater)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        Log.d(TAG,"onCreate called")
        setContentView(R.layout.activity_main)

        val currentIndex = savedInstanceState?.getInt(KEY_INDEX,0)?:0
        quizModel.currentIndex=currentIndex

        quizModel.isCheater = savedInstanceState?.getBoolean(IS_CHEATER,false)?:false

        trueButton = findViewById(R.id.true_button)
        falseButton = findViewById(R.id.false_button)
        nextButton = findViewById(R.id.next_button)
        questionTextView = findViewById(R.id.question_text_view)
        cheatButton = findViewById(R.id.cheat_button)
        cheatButton.setOnClickListener {
            val answerIsTrue = quizModel.currentQuestionAnswer
            val intent = CheatActivity.newIntent(this@MainActivity,answerIsTrue)
            startActivityForResult(intent, REQUEST_CODE_CHEAT)
        }
        trueButton.setOnClickListener { view ->
            checkAnswer(true)
        }
        falseButton.setOnClickListener { view ->
            checkAnswer(false)
        }
        nextButton.setOnClickListener {
            quizModel.isCheater=false
            quizModel.moveToNextQuestion()
            updateQuestion()
        }
        updateQuestion()
    }

    override fun onStart() {
        super.onStart()
        Log.d(TAG,"onStart called")
    }
    override fun onResume() {
        super.onResume()
        Log.d(TAG,"onResume called")
    }
    override fun onPause() {
        super.onPause()
        Log.d(TAG,"onPause called")
    }
    override fun onDestroy() {
        super.onDestroy()
        Log.d(TAG,"onDestroy called")
    }
    override fun onStop() {
        super.onStop()
        Log.d(TAG,"onStop called")
    }
    private fun updateQuestion(){
        val questionTextResId = quizModel.currentQuestionText
        questionTextView.setText(questionTextResId)
    }
    private fun checkAnswer(userAnswer:Boolean){
        val actualAnswer = quizModel.currentQuestionAnswer
        val message = when{
            quizModel.isCheater->getString(R.string.judgement_toast)
            (actualAnswer == userAnswer) -> getString(R.string.correct_toast)
            else -> getString(R.string.incorrect_toast)
        }
        Toast.makeText(this, message, Toast.LENGTH_SHORT).show()
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        if(resultCode != Activity.RESULT_OK) return
        if(requestCode== REQUEST_CODE_CHEAT){
            quizModel.isCheater = data?.getBooleanExtra(EXTRA_ANSWER_SHOWN,false)?:false
        }
    }
}

CheatActivity.kt

private const val EXTRA_ANSWER_IS_TRUE = "answer_is_true"
private const val EXTRA_ANSWER_SHOWN = "answer_is_shown"
private const val TAG = "CheatActivity"
private const val IS_CHEATER = "isCheater"
class CheatActivity : AppCompatActivity() {

    private var answerIsTrue = false
    private lateinit var answerText: TextView
    private lateinit var showAnswer: Button

    private val quizModel: QuizViewModel by lazy {
        ViewModelProviders.of(this).get(QuizViewModel::class.java)
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        Log.i(TAG,"onSaveInstanceState")
        outState.putBoolean(IS_CHEATER,quizModel.isCheater)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_cheat)
        answerIsTrue = intent.getBooleanExtra(EXTRA_ANSWER_IS_TRUE,false)
        answerText = findViewById(R.id.answer_text_view)
        showAnswer = findViewById(R.id.show_answer_button)
        showAnswer.setOnClickListener {
            answerText.text = displayAnswer
            setAnswerShownResult(true)
        }
        if(quizModel.isCheater){
            answerText.text = displayAnswer
            setAnswerShownResult(true)
        }
    }
    private val displayAnswer:String
        get() =when{
            answerIsTrue -> getString(R.string.true_button)
            else -> getString(R.string.false_button)
        }
    companion object{
        fun newIntent(packageContext: Context, answerIsTrue:Boolean): Intent {
            return Intent(packageContext, CheatActivity::class.java).apply { putExtra(
                EXTRA_ANSWER_IS_TRUE,answerIsTrue) }
        }
    }
    private fun setAnswerShownResult(isAnswerShown:Boolean){
        val data = Intent().apply { putExtra(EXTRA_ANSWER_SHOWN,isAnswerShown) }
        setResult(Activity.RESULT_OK,data)
    }
}

Please let me know for any errors.