ActivityMainBinding 에러

package com.bokchi.databinding_ex

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Button
import android.widget.Toast
import com.bokchi.databinding_ex.databinding.ActivityMainBinding


class MainActivity : AppCompatActivity() {

    private  lateinit var binding : ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val btn = findViewById<Button>(R.id.testBtnID)
        btn.setOnClickListener{
            Toast.makeText(this, "click", Toast.LENGTH_LONG).show()
        }
    }
}

이렇게 적었는데 에러가 나요..왼도우 L 눌렀는데도 안떠서 적었는데도 빨간색으로 에러만 뜸니다.....