I am very new to Android Development.
I am currently making a calculator with Kotlin in android studio.
I am have currently 9 buttons with numerical values(1, 2, 3, 4, 5, 6, 7, 8, 9)
I want when i click the button the text on the textView changes according to the values on the button. When i click the button the text on textView changes, but, there is strange bug. Like, when i click on 7 first time it does not add the text, and then if i click on some other button like 8 instead of adding 8 it adds 7 to the textView and then if i click on say 5, it adds 8 instead of 5. My MainActivity.kt code is :-

```
package com.example.calculator

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

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        num1.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "1")
        }

        num2.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "2")
        }

        num3.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "3")
        }

        num4.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "4")
        }

        num5.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "5")
        }

        num6.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "6")
        }

        num7.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "7")
        }

        num8.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "8")
        }

        num9.setOnClickListener {
            mainText.setText(mainText.getText().toString() + "9")
        }

    }

}

```

and my activity\_main.xml code is :-

```
<?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">

    <TextView
        android:id="@+id/mainText"
        android:layout_width="408dp"
        android:layout_height="68dp"
        android:background="#f2f2f2"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.333"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:textDirection="rtl"
        android:paddingRight="20sp"
        android:textSize="25sp"/>

    <!-- First Row-->
    <Button
        android:id="@+id/num9"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:text="9"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/num8"
        app:layout_constraintTop_toTopOf="@+id/num8" />

    <Button
        android:id="@+id/num8"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:layout_marginStart="8dp"
        android:text="8"
        app:layout_constraintBottom_toBottomOf="@+id/num7"
        app:layout_constraintStart_toEndOf="@+id/num7" />

    <Button
        android:id="@+id/num7"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:layout_marginStart="10dp"
        android:layout_marginTop="10dp"
        android:text="7"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/mainText" />

    <!-- Second Row -->

    <Button
        android:id="@+id/num6"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:text="6"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/num5"
        app:layout_constraintTop_toTopOf="@+id/num5" />

    <Button
        android:id="@+id/num5"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:layout_marginStart="10dp"
        android:text="5"
        app:layout_constraintBottom_toBottomOf="@+id/num4"
        app:layout_constraintStart_toEndOf="@+id/num4" />

    <Button
        android:id="@+id/num4"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="10dp"
        android:text="4"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/num7" />

    <!-- Third Row -->

    <Button
        android:id="@+id/num3"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:text="3"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toEndOf="@+id/num2"
        app:layout_constraintTop_toTopOf="@+id/num2" />

    <Button
        android:id="@+id/num2"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:layout_marginStart="10dp"
        android:text="2"
        app:layout_constraintBottom_toBottomOf="@+id/num1"
        app:layout_constraintStart_toEndOf="@+id/num1" />

    <Button
        android:id="@+id/num1"
        android:layout_width="110dp"
        android:layout_height="54dp"
        android:layout_marginStart="10dp"
        android:layout_marginTop="10dp"
        android:text="1"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/num4" />

</androidx.constraintlayout.widget.ConstraintLayout>

```

I am very confused why this bug is occuring and what is the problem in my code.
Thanks in Advance :-