.adapter부분이 빨간색이 떠요. 어떤게 빠진건가요?

ChatListActivity.kt

package com.example.chatting_video

import Model.UserItem
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.xwray.groupie.GroupAdapter
import com.xwray.groupie.GroupieViewHolder
import kotlinx.android.synthetic.main.activity_chat_list.*

class ChatListActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_chat_list)

        val adapter = GroupAdapter<GroupieViewHolder>()

        adapter.add(UserItem())
        adapter.add(UserItem())
        adapter.add(UserItem())

        recyclerview_list.adapter = adapter



    }
}

.adapter부분이 빨간색이 떠요. = adapter는 괜찮아요.
adapter부분에 뭐가 빠져서 빨간색이 뜨는것 같아요.
e: C:\Users\MS\AndroidStudioProjects\chatting_video\app\src\main\java\com\example\chatting_video\ChatListActivity.kt: (21, 27): Unresolved reference: adapter

UserItem.kt

package Model

import android.content.ClipData
import com.example.chatting_video.R
import com.xwray.groupie.GroupieViewHolder
import com.xwray.groupie.Item

class UserItem : Item<GroupieViewHolder>() {
    override fun getLayout(): Int {
        return R.layout.message_list_row
    }

    override fun bind(viewHolder: GroupieViewHolder, position: Int) {


    }
}

activity_chat_list.xml

<?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:id="@+id/recyclerview_list"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ChatListActivity">

    <androidx.recyclerview.widget.RecyclerView
        android:layout_width="344dp"
        android:layout_height="572dp"
        android:layout_marginStart="20dp"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="40dp"
        android:layout_marginEnd="20dp"
        android:layout_marginRight="20dp"
        android:layout_marginBottom="40dp"
        app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.761"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.632" />
</androidx.constraintlayout.widget.ConstraintLayout>