android kotlin - Get RadioGroup selected item

MainActivity.kt

package com.example.jetpack

import android.os.Bundle
import android.widget.RadioButton
import androidx.appcompat.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*


class MainActivity : AppCompatActivity() {

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

        button.setOnClickListener {
            // get radio group checked / selected item id
            val id  = radioGroup.checkedRadioButtonId
            when(id){
                -1->{ // return -1 when none selected
                    textView2.text = "Nothing selected."
                }else->{
                    val selectedItem = findViewById<RadioButton>(id)
                    // show the checked / selected item text in text view
                    textView2.text = selectedItem.text
                }
            }
        }
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:background="#F5F5F5">

    <TextView
        android:id="@+id/textView"
        style="@style/TextAppearance.MaterialComponents.Headline6"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:padding="8dp"
        android:text="Which is your most favorite?"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <RadioGroup
        android:id="@+id/radioGroup"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="32dp"
        android:layout_marginTop="2dp"
        android:layout_marginEnd="16dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView">

        <com.google.android.material.radiobutton.MaterialRadioButton
            android:id="@+id/radioFlash"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Flash" />

        <com.google.android.material.radiobutton.MaterialRadioButton
            android:id="@+id/radioFlex"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Flex" />

        <com.google.android.material.radiobutton.MaterialRadioButton
            android:id="@+id/radioColdFusion"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="ColdFusion" />

        <com.google.android.material.radiobutton.MaterialRadioButton
            android:id="@+id/radioPhotoshop"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Photoshop" />

    </RadioGroup>

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        style="@style/TextAppearance.MaterialComponents.Headline5"
        android:textStyle="bold"
        android:layout_marginTop="16dp"
        android:textColor="#483D8B"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.547"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button"
        tools:text="TextView" />

    <com.google.android.material.button.MaterialButton
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="Get Checked Item Text"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/radioGroup" />

</androidx.constraintlayout.widget.ConstraintLayout>