I am trying to use TextToSpeech in an Activity.
And I also want to move this to a seperate class file called Helper, and load it from there.
So I created the code below.

// Helper.kt
class Helper {
    private var tts: TextToSpeech? = null

    fun initTextToSpeech(context: Context) {
        // 롤리팝(API 21, Android 5.0)이상에서만 지원됨
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            Toast.makeText(context, "지원하지 않는 SDK 버전입니다.", Toast.LENGTH_SHORT).show()
            return
        }

        tts = TextToSpeech(context) {
            if (it == TextToSpeech.SUCCESS) {
                val result = tts?.setLanguage(Locale.KOREAN)
                if (result == TextToSpeech.LANG_MISSING_DATA || result == TextToSpeech.LANG_NOT_SUPPORTED) {
                    Toast.makeText(context, "지원하지 않는 언어입니다.", Toast.LENGTH_SHORT).show()
//                    return@TextToSpeech
                }
                Toast.makeText(context, "TTS 설정 성공", Toast.LENGTH_SHORT).show()
            } else {
                Toast.makeText(context, "TTS 설정 실패", Toast.LENGTH_SHORT).show()
            }
        }
    }

    fun ttsSpeak(str: String) {
        tts?.speak(str, TextToSpeech.QUEUE_FLUSH, null, "")
    }
}

I called it like this in the activity as below, but it didn’t work.
There is no error, but it does not work either.

class QuizQuestionsActivity : AppCompatActivity(), View.OnClickListener {

    private var btnAudio: ImageButton? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_quiz_questions)

        Helper().initTextToSpeech(this)

        btnAudio = findViewById(R.id.btn_audio)
        btnAudio?.setOnClickListener(this)

    }

    override fun onClick(view: View?) {

        // 클릭된 view 요소에 따라 메서드 실행
        when (view?.id) {
            R.id.btn_audio -> {
                val question = mQuestionList?.get(mCurrentPosition - 1)
                question?.let { Helper().ttsSpeak(question.question) }
            }
        }
    }
}

It works well if I include it in the Activity itself as below.
(* Please note that some parts that are not directly related have been omitted.)

// QuizQuestionsActivity.kt

class QuizQuestionsActivity : AppCompatActivity(), View.OnClickListener {
   
   private var btnAudio: ImageButton? = null
   var tts: TextToSpeech? = null

   override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_quiz_questions)

        initTextToSpeech()

        btnAudio = findViewById(R.id.btn_audio)
        btnAudio?.setOnClickListener(this)  
   }

   fun initTextToSpeech() {
        // 롤리팝(API 21, Android 5.0)이상에서만 지원됨
        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
            Toast.makeText(this, "지원하지 않는 SDK 버전입니다.", Toast.LENGTH_SHORT).show()
            return
        }

        tts = TextToSpeech(this) {
            if (it == TextToSpeech.SUCCESS) {
                val result = tts?.setLanguage(Locale.KOREAN)
                if (result == TextToSpeech.LANG_MISSING_DATA || result == TextToSpeech.LANG_NOT_SUPPORTED) {
                    Toast.makeText(this, "지원하지 않는 언어입니다.", Toast.LENGTH_SHORT).show()
//                    return@TextToSpeech
                }
                Toast.makeText(this, "TTS 설정 성공", Toast.LENGTH_SHORT).show()
            } else {
                Toast.makeText(this, "TTS 설정 실패", Toast.LENGTH_SHORT).show()
            }
        }
    }

    fun ttsSpeak(str: String) {
        tts?.speak(str, TextToSpeech.QUEUE_ADD, null, "")
    }

     override fun onClick(view: View?) {

        // 클릭된 view 요소에 따라 메서드 실행
        when (view?.id) {
            R.id.btn_audio -> {
                val question = mQuestionList?.get(mCurrentPosition - 1)
                question?.let { ttsSpeak(question.question) }
            }
        }
    }
}

I’m still new to Kotlin, so I don’t know the cause.
Why the first one doesn’t work and how can I make it work?
I don’t understand because actually there are other methods in the Helper class and they works fine in the activity.