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);
    }
}

In your function “RequestBanner”
you have

BannerView bannerView = new BannerView(adUnitId, AdSize.SmartBanner, AdPosition.Bottom);

You dont need to declare again ‘bannerView’, in other words, remove ‘BannerView’. Because if not is different "bannerView "that you are using in start and is other “bannerView” in “RequestBanner”. So when “RequestBanner” ends, in start “bannerView” is still null.