Ошибка в коде (onCreate) в Android Studio

Как исправить ошибку?
введите сюда код

 RadioGroup radioGroup;
    // это будет именем файла настроек
    public static final String APP_PREFERENCES = "mysettings";
    final String KEY_RADIOBUTTON_INDEX = "SAVED_RADIO_BUTTON_INDEX";

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

        radioGroup = findViewById(R.id.sex_RadioGroup);
        radioGroup
                .setOnCheckedChangeListener(radioGroupOnCheckedChangeListener); //ошибка в этой строчке

        LoadPreferences();
    }

    RadioGroup.OnCheckedChangeListener radioGroupOnCheckedChangeListener = new RadioGroup.OnCheckedChangeListener() {

        @Override
        public void onCheckedChanged(RadioGroup group, int checkedId) {

            RadioButton checkedRadioButton = (RadioButton) radioGroup
                    .findViewById(checkedId);
            int checkedIndex = radioGroup.indexOfChild(checkedRadioButton);

            SavePreferences(KEY_RADIOBUTTON_INDEX, checkedIndex);
        }
    };

    private void SavePreferences(String key, int value) {
        SharedPreferences sharedPreferences = getSharedPreferences(
                APP_PREFERENCES, MODE_PRIVATE);
        SharedPreferences.Editor editor = sharedPreferences.edit();
        editor.putInt(key, value);
        editor.apply();
    }

    private void LoadPreferences() {
        SharedPreferences sharedPreferences = getSharedPreferences(
                APP_PREFERENCES, MODE_PRIVATE);
        int savedRadioIndex = sharedPreferences.getInt(
                KEY_RADIOBUTTON_INDEX, 0);
        RadioButton savedCheckedRadioButton = (RadioButton) radioGroup
                .getChildAt(savedRadioIndex);
        savedCheckedRadioButton.setChecked(true);
    }

Ошибка: at com.whattowear.MainActivity.onCreate(MainActivity.java:35) UPD: ошибку так же отметил в коде

2020-02-13 20:07:01.706 1954-1954/? E/AndroidRuntime: FATAL EXCEPTION: main Process: com.whattowear, PID: 1954 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.whattowear/com.whattowear.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RadioGroup.setOnCheckedChangeListener(android.widget.RadioGroup$OnCheckedChangeListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2977) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3114) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:71) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1859) at android.os.Handler.dispatchMessage(Handler.java:106) at android.os.Looper.loop(Looper.java:201) at android.app.ActivityThread.main(ActivityThread.java:6831) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:927) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RadioGroup.setOnCheckedChangeListener(android.widget.RadioGroup$OnCheckedChangeListener)' on a null object reference at com.whattowear.MainActivity.onCreate(MainActivity.java:35) at android.app.Activity.performCreate(Activity.java:7224) at android.app.Activity.performCreate(Activity.java:7213) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3114)  at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)  at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:113)  at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:71)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1859)  at android.os.Handler.dispatchMessage(Handler.java:106)  at android.os.Looper.loop(Looper.java:201)  at android.app.ActivityThread.main(ActivityThread.java:6831)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:927)