Insert Multiple Data in Kotlin

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.realtimedatabasekotlin">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.RealtimedatabaseKotlin">
        <activity android:name=".UpdateData">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>


        </activity>
        <activity android:name=".ReadData">

        </activity>
        <activity android:name=".MainActivity"></activity>
    </application>

</manifest>

package com.example.realtimedatabasekotlin

import android.os.Binder
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import com.example.realtimedatabasekotlin.databinding.ActivityMainBinding
import com.google.firebase.database.DatabaseReference
import com.google.firebase.database.FirebaseDatabase

class MainActivity : AppCompatActivity() {

    private lateinit var binding : ActivityMainBinding
    private lateinit var database : DatabaseReference
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        binding.registerBtn.setOnClickListener {

            val firstName = binding.firstName.text.toString()
            val lastName = binding.lastName.text.toString()
            val age = binding.age.text.toString()
            val userName = binding.userName.text.toString()

            database = FirebaseDatabase.getInstance().getReference("Users")
            val User = User(firstName,lastName,age,userName)
            database.child(userName).setValue(User).addOnSuccessListener {

                binding.firstName.text.clear()
                binding.lastName.text.clear()
                binding.age.text.clear()
                binding.userName.text.clear()

                Toast.makeText(this,"Successfully Saved",Toast.LENGTH_SHORT).show()

            }.addOnFailureListener{

                Toast.makeText(this,"Failed",Toast.LENGTH_SHORT).show()


            }


        }

    }
}








Model Class:-


package com.example.schooladmin.ui.home.model
data class User(val firstName : String,val lastName : String,val age : String,val userName : String){


}