Read data from firebase in arraylist and show on Recycler view in kotlin android

Question

database click here to see database image

my data is uploaded on firebase i want to read my data and add in arraylist dataset and then shown in recycler view but my app crashed and i think it is due to pass of null length array and i am new to kotlin but i think that below approach is workable in java help to find out the error or suggest me method.

Main Activity

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

    var dataSet = ArrayList<User>()
    var database = FirebaseDatabase.getInstance()
    var usersAdapter= updateAdapter(dataSet)
    var recyclerView = findViewById<RecyclerView>(R.id.recyclerViewId)
    recyclerView.adapter=usersAdapter

    database.getReference().child("Users").addValueEventListener(object: ValueEventListener {
        override fun onDataChange(snapshot: DataSnapshot) {
            Log.d("MAIN_ACTIVITY","onDataChange called")
            dataSet.clear()
            for(snapshot1: DataSnapshot in snapshot.children){
                var user: User? = snapshot1.getValue<User>()
                dataSet.add(user!!)
            }
            usersAdapter.notifyDataSetChanged()
        }

        override fun onCancelled(error: DatabaseError) {
            Log.d("MAIN_ACTIVITY","onCancelled called")
        }
    })

}

updateAdapter

class updateAdapter(var dataset: ArrayList): RecyclerView.Adapter<updateAdapter.userViewHolder>() {

class userViewHolder(view: View): RecyclerView.ViewHolder(view) {
    var username = view.findViewById<TextView>(R.id.username)
    var lastMsg = view.findViewById<TextView>(R.id.lastMsg)
    var msgTime = view.findViewById<TextView>(R.id.msgTime)
    var profile = view.findViewById<ImageView>(R.id.profile)
}

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): userViewHolder {
    Log.d("MAIN_ACTIVITY","onCreate View HOlder")
    var view = LayoutInflater.from(parent.context).inflate(R.layout.row_conversation,parent,false)
    return userViewHolder(view)
}

override fun onBindViewHolder(holder: userViewHolder, position: Int) {
    Log.d("MAIN_ACTIVITY","onBLind ViewHolder")

    var user = dataset[position]
    holder.username.text = user.Uname
}

override fun getItemCount(): Int {
    return dataset.size
}

}

User

  data class User(
    var Uid :String?="",
    var Uname:String?="",
    var Unumber :String?= "",
    var Uimage :String?="")

ERROR

  E/Perf: Fail to get file list com.example.ranka
  E/Perf: getFolderSize() : Exception_1 = java.lang.NullPointerException: Attempt to get length          of null array
Fail to get file list com.example.ranka
getFolderSize() : Exception_1 = java.lang.NullPointerException: Attempt to get length of null array

ERROR

 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ranka, PID: 27503
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.ranka/com.example.ranka.Activities.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.ranka.Activities.MainActivity> cannot be instantiated
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3404)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3636)
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:140)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:100)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2222)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:228)
    at android.app.ActivityThread.main(ActivityThread.java:7782)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:981)
 Caused by: java.lang.InstantiationException: java.lang.Class<com.example.ranka.Activities.MainActivity> cannot be instantiated
    at java.lang.Class.newInstance(Native Method)
    at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
    at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1251)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3392)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3636) 
    at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
    at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:140) 
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:100) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2222) 
    at android.os.Handler.dispatchMessage(Handler.java:107) 
    at android.os.Looper.loop(Looper.java:228) 
    at android.app.ActivityThread.main(ActivityThread.java:7782) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:981) 

I/Process: Sending signal. PID: 27503 SIG: 9