非活动类/视图适配器类中的TextToSpeech失败:未绑定到TTS引擎

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.MODIFY_AUDIO_SETTINGS" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_PHONE_STATE" />

TextToSpeech tts;

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

    tts = new TextToSpeech(this, new TextToSpeech.OnInitListener() {
        @Override
        public void onInit(int status) {
            if (status == TextToSpeech.SUCCESS) {
                // TTS引擎初始化成功
                int result = tts.setLanguage(Locale.US);
                if (result == TextToSpeech.LANG_MISSING_DATA || result == TextToSpeech.LANG_NOT_SUPPORTED) {
                    // 不支持所选语言
                    Log.e("TTS", "Language not supported");
                }
            } else {
                // TTS引擎初始化失败
                Log.e("TTS", "Initialization failed");
            }
        }
    });
}

@Override
protected void onResume() {
    super.onResume();
    if (tts != null) {
        int result = tts.setLanguage(Locale.US);
        if (result == TextToSpeech.LANG_MISSING_DATA || result == TextToSpeech.LANG_NOT_SUPPORTED) {
            // 不支持所选语言
            Log.e("TTS", "Language not supported");
        }
    }
}

@Override
protected void onDestroy() {
    super.onDestroy();
    if (tts != null) {
        tts.stop();
        tts.shutdown();
    }
}