Why am I getting this error with my BannerAD (Admob)

This is the error i'm getting : "Assets/Scripts/BannerAds.cs(7,24): warning CS0649: Field BannerAds.bannerView' is never assigned to, and will always have its default value null'" this is my script:

 using UnityEngine;
 using System.Collections;
 using GoogleMobileAds.Api;
 
 public class BannerAds : MonoBehaviour
 {
     private BannerView bannerView;
 
     void Start()
     {
         RequestBanner();
         bannerView.Show();
     }
 
     void Update()
     {
         if (Application.loadedLevelName == "Level")
         {
             bannerView.Hide();
             bannerView.Destroy();
         }
     }
 
     private void RequestBanner()
     {
        #if UNITY_ANDROID
         string adUnitId = "ca-app-pub-xxxxxxxxxxxxxxxxxxxxxx";
        #elif UNITY_IPHONE
         string adUnitId = "INSERT_IOS_BANNER_AD_UNIT_ID_HERE";
        #else
         string adUnitId = "unexpected_platform";
        #endif
 
         // Create a 320x50 banner at the top of the screen.
         BannerView bannerView = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Bottom);
         // Create an empty ad request.
         AdRequest request = new AdRequest.Builder().Build();
         // Load the banner with the request.
         bannerView.LoadAd(request);
     }
 }