NullReferenceException: Object reference not set to an instance of an object Admanager.ShowFullScreenAd ()

Hi I am trying to implement the admob interstitial ad in my project . I had created an empty game object called Admanager in the scene and attached the Admanager.cs script to it. I wanted to show up an interstitial ad whenever the player clicks the replay button. But when I click the replay button it is showing as NullReferenceException: Object reference not set to an instance of an object Admanager.ShowFullScreenAd () and indicating that there is an error in this line if (this.interstitial.IsLoaded()) . I don't Understand what mistake i am doing

I had attached the Admanager.cs script and RestartMenu.cs (where i call the interstitial ad to show up).

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using GoogleMobileAds.Api;
 using System;
 using System.Diagnostics;
 using Debug = UnityEngine.Debug;
 
 public class Admanager : MonoBehaviour
 {
     public static Admanager instance;
     private string appID = " APP-ID";
     public InterstitialAd interstitial;
     private string fullScreenAdID = "INTERSTITIAL- ID";
 
     public void Awake()
     {
         if(instance == null)
         {
             instance = this;
         }
         else
         {
             Destroy(this);
         }
     }
     private void start()
     { 
         MobileAds.Initialize(appID);
         RequestInterstitial();
     }
 
     public void RequestInterstitial()
     {
         // Initialize an InterstitialAd.
         this.interstitial = new InterstitialAd(fullScreenAdID);
 
         // Called when an ad request has successfully loaded.
         this.interstitial.OnAdLoaded += HandleOnAdLoaded;
         // Called when an ad request failed to load.
         this.interstitial.OnAdFailedToLoad += HandleOnAdFailedToLoad;
         // Called when an ad is shown.
         this.interstitial.OnAdOpening += HandleOnAdOpened;
         // Called when the ad is closed.
         this.interstitial.OnAdClosed += HandleOnAdClosed;
         // Called when the ad click caused the user to leave the application.
         this.interstitial.OnAdLeavingApplication += HandleOnAdLeavingApplication;
 
         // Create an empty ad request.
         AdRequest request = new AdRequest.Builder().Build();
         // Load the interstitial with the request.
         this.interstitial.LoadAd(request);
     }
 
     public void ShowFullScreenAd()
     {
         if (this.interstitial.IsLoaded())
         {
             this.interstitial.Show();
             RequestInterstitial();
         }
         else
         {
             Debug.Log("not ready yet");
             RequestInterstitial();
         }
     }
 
     public void HandleOnAdLoaded(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdLoaded event received");
     }
 
     public void HandleOnAdFailedToLoad(object sender, AdFailedToLoadEventArgs args)
     {
         MonoBehaviour.print("HandleFailedToReceiveAd event received with message: "
                             + args.Message);
     }
 
     public void HandleOnAdOpened(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdOpened event received");
     }
 
     public void HandleOnAdClosed(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdClosed event received");
     }
 
     public void HandleOnAdLeavingApplication(object sender, EventArgs args)
     {
         MonoBehaviour.print("HandleAdLeavingApplication event received");
     }
 }

RestartMenu.cs script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class RestartMenu : MonoBehaviour
 {
     public void menu()
     {
         FindObjectOfType<AudioManager>().Play("click");
         SceneManager.LoadScene(0);
     }
     public void replay()
     {
         FindObjectOfType<AudioManager>().Play("click");
         Admanager.instance.ShowFullScreenAd();
         SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);
     }
 }

,