emailArea가 안나와요..

package com.bokchi.mysolelife.auth

import android.database.DatabaseUtils
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.Toast
import androidx.databinding.DataBindingUtil
import com.bokchi.mysolelife.R
import com.bokchi.mysolelife.databinding.ActivityIntroBinding
import com.google.firebase.Firebase
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.auth.auth

class JoinActivity : AppCompatActivity() {

    private lateinit var binding: ActivityIntroBinding

    private lateinit var auth: FirebaseAuth

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)
        auth = Firebase.auth

        binding = DataBindingUtil.setContentView(this, R.layout.activity_join)

        binding.joinBtn.setOnClickListener {
            val email = binding.emailArea
        }


//        auth.createUserWithEmailAndPassword("abc@abc.com", "12341234")
//            .addOnCompleteListener(this) { task ->
//                if (task.isSuccessful) {
//                    Toast.makeText(this, "성공", Toast.LENGTH_LONG).show()
//                } else {
//                    Toast.makeText(this, "실패", Toast.LENGTH_LONG).show()
//                }
//            }
    }
}

여기 보면 선생님과 똑같이 했는데 emailArea을 이제 칠때 빨간색으로 뜨면서 안되요...

<?xml version="1.0" encoding="utf-8"?>
<layout>
    <LinearLayout
        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=".auth.JoinActivity"
        android:orientation="vertical">

        <LinearLayout
            android:layout_width="match_parent"
            android:background="@color/mainColor"
            android:layout_height="150dp">

            <TextView
                android:text="회원가입"
                android:gravity="center"
                android:textStyle="bold"
                android:textColor="@color/black"
                android:textSize="20sp"
                android:layout_width="match_parent"
                android:layout_height="50dp"/>
        </LinearLayout>

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginTop="50dp"
            android:orientation="vertical">

            <EditText
                android:id="@+id/emailArea"
                android:layout_width="match_parent"
                android:layout_height="50dp"
                android:hint="email"
                style="@style/AuthEditText"/>

            <LinearLayout
                android:layout_width="match_parent"
                android:background="#999999"
                android:layout_marginLeft="10dp"
                android:layout_marginRight="10dp"
                android:layout_height="0.5dp"/>

            <EditText
                android:id="@+id/passwordArea"
                android:layout_width="match_parent"
                android:layout_height="50dp"
                android:inputType="textPassword"
                android:hint="password"
                style="@style/AuthEditText"/>

            <LinearLayout
                android:layout_width="match_parent"
                android:background="#999999"
                android:layout_marginLeft="10dp"
                android:layout_marginRight="10dp"
                android:layout_height="0.5dp"/>

            <EditText
                android:id="@+id/passwordArea2"
                android:layout_width="match_parent"
                android:layout_height="50dp"
                android:inputType="textPassword"
                android:hint="password check"
                style="@style/AuthEditText"/>

            <LinearLayout
                android:layout_width="match_parent"
                android:background="#999999"
                android:layout_marginLeft="10dp"
                android:layout_marginRight="10dp"
                android:layout_height="0.5dp"/>
            <Button
                android:id="@+id/joinBtn"
                android:layout_margin="20dp"
                android:background="@drawable/background_radius_yellow"
                android:text="회원가입하기"
                android:layout_width="match_parent"
                android:layout_height="50dp"/>
        </LinearLayout>


    </LinearLayout>
</layout>

여기 ID emailArea 적어져 있습니다 그런데 왜(?) 이런 오류가 뜨는 것일가요?? 알려주세요...ㅠㅠ