Object reference not set to an instance of an object

Hi, when I want to add AdMob Interstitial ads for test I get this error
"NullReferenceException: Object reference not set to an instance of an object"


Here is my code:

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. using GoogleMobileAds.Api;
5.  
6. public class GoogleFullScreen : MonoBehaviour
7. {
8.     private InterstitialAd interstitial;
9.  
10.     void Start()
11.     {
12.         if (this.interstitial.IsLoaded())
13.         {
14.             this.interstitial.Show();
15.         }
16.     }
17.  
18.     private void RequestInterstitial()
19.     {
20.     #if UNITY_ANDROID
21.         string adUnitId = "ca-app-pub-3940256099942544/1033173712";
22.     #elif UNITY_IPHONE
23.         string adUnitId = "ca-app-pub-3940256099942544/4411468910";
24.     #else
25.         string adUnitId = "unexpected_platform";
26.     #endif
27.  
28.     // Initialize an InterstitialAd.
29.     this.interstitial = new InterstitialAd(adUnitId);
30.     // Create an empty ad request.
31.     AdRequest request = new AdRequest.Builder().Build();
32.     // Load the interstitial with the request.
33.     this.interstitial.LoadAd(request);
34.     }
35. }
36.  

 

"NullReferenceException: Object reference not set to an instance of an object
GoogleFullScreen.Start () (at Assets/GoogleFullScreen.cs:12)"

 

thank you, it works