Volley JsonObjectRequest in Kotlin

package com.jigopost.volley

import android.os.Bundle
import android.text.method.ScrollingMovementMethod
import android.view.View
import androidx.appcompat.app.AppCompatActivity
import com.android.volley.Request
import com.android.volley.toolbox.JsonObjectRequest
import kotlinx.android.synthetic.main.activity_main.*
import org.json.JSONException
import org.json.JSONObject


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

        // make text view content scrollable
        textView.movementMethod = ScrollingMovementMethod()

        // url to get json object
        val url = "https://pastebin.com/raw/2bW31yqa"


        // get json object from url using volley network library
        button.setOnClickListener {
            progressBar.visibility = View.VISIBLE

            // request json object response from the provided url
            val request = JsonObjectRequest(
                Request.Method.GET, // method
                url, // url
                null, // json request
                { response -> // response listener

                    try {
                        val obj: JSONObject = response
                        val array = obj.getJSONArray("students")

                        textView.text = ""

                        // loop through the array elements
                        for (i in 0 until  array.length()){
                            // get current json object as student instance
                            val student: JSONObject = array.getJSONObject(i)

                            // get the current student (json object) data
                            val firstName: String = student.getString("firstname")
                            val lastName: String = student.getString("lastname")
                            val age: Int = student.getInt("age")

                            //display the formatted json data in text view
                            textView.append("$firstName $lastName\nage : $age\n\n")
                        }

                    }catch (e: JSONException){
                        textView.text = e.message
                    }

                    progressBar.visibility = View.INVISIBLE
                },
                { error -> // error listener
                    textView.text = error.message
                    progressBar.visibility = View.INVISIBLE
                }
            )

            // add network request to volley queue
            VolleySingleton.getInstance(applicationContext)
                .addToRequestQueue(request)
        }
    }
}

package com.jigopost.volley

import android.content.Context
import android.graphics.Bitmap
import android.util.LruCache
import com.android.volley.Request
import com.android.volley.RequestQueue
import com.android.volley.toolbox.ImageLoader
import com.android.volley.toolbox.Volley


class VolleySingleton constructor(context: Context) {
    companion object {
        @Volatile
        private var INSTANCE: VolleySingleton? = null
        fun getInstance(context: Context) =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: VolleySingleton(context).also {
                    INSTANCE = it
                }
            }
    }
    val imageLoader: ImageLoader by lazy {
        ImageLoader(requestQueue,
            object : ImageLoader.ImageCache {
                private val cache = LruCache<String, Bitmap>(20)
                override fun getBitmap(url: String): Bitmap {
                    return cache.get(url)
                }
                override fun putBitmap(url: String, bitmap: Bitmap) {
                    cache.put(url, bitmap)
                }
            })
    }
    private val requestQueue: RequestQueue by lazy {
        // applicationContext is key, it keeps you from leaking the
        // Activity or BroadcastReceiver if someone passes one in.
        Volley.newRequestQueue(context.applicationContext)
    }
    fun <T> addToRequestQueue(req: Request<T>) {
        requestQueue.add(req)
    }
}

<?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:id="@+id/constraintLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#EDEAE0"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:text="Get Json Object"
        android:textAllCaps="false"
        android:backgroundTint="#3B2F2F"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyle"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:visibility="invisible"
        app:layout_constraintBottom_toBottomOf="@+id/button"
        app:layout_constraintStart_toEndOf="@+id/button"
        app:layout_constraintTop_toTopOf="@+id/button" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="12dp"
        android:layout_marginEnd="8dp"
        android:fontFamily="sans-serif-condensed-medium"
        android:padding="8dp"
        android:textColor="#4F42B5"
        android:textSize="22sp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/button"
        tools:text="TextView" />

</androidx.constraintlayout.widget.ConstraintLayout>

// volley network library
implementation 'com.android.volley:volley:1.1.1'