AdMob firebase destroy banner issue

Hi,

This is my first time using admob. I want to show a banner when the player dies, and then destroy it when he reloads the game. Ideally I have a method for calling the ad, and another to destroy it, but bannerView doesn't exists globally when I follow their example, and I'm not sure how to make it work.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using GoogleMobileAds;
 using GoogleMobileAds.Api;
 
 public class GoogleAds : MonoBehaviour {
 
     private int noAdsBought; // is the no ads iap bought
 
     void Start () {                    
     }    
 
     public void RollBanner ()
     {
         noAdsBought = PlayerPrefManager.GetAreNoAds();    
 
         if (noAdsBought == 0) {
             #if UNITY_EDITOR
             string adUnitId = "unused";
             #elif UNITY_ANDROID
                 string adUnitId = "ca-app-pub-5420374779693849/7702545613";
             #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.Banner, AdPosition.TopRight);
             // Create an empty ad request.
             AdRequest request = new AdRequest.Builder ().Build ();
             // Load the banner with the request.
             bannerView.LoadAd (request);
             } else if (noAdsBought == 1) {
             print ("no ads because no ads iap bought");
         }    
     }
 
     public void DestroyBanner ()
     {
             bannerView.Destroy();
     }
 }
 

Thanks!