Help: Game crashing when opening in android

It's my first time making a game with unity and to put ads in my game i followed youtube and wrote down the following code. In the Unity editor after build the message like below pops up and after importing to phone it just crashes... Any help?

Code (CSharp):

1. using GoogleMobileAds.Api;
2. using System.Collections;
3. using System.Collections.Generic;
4. using UnityEngine;
5.  
6. public class AdScript : MonoBehaviour {
7.  
8.     // Use this for initialization
9.     void Start () {
10.  
11.         showBannerAd();
12.        
13.     }
14.  
15.     private void showBannerAd()
16.     {
17.         string adID = "ca-app-pub-3940256099942544/6300978111";
18.  
19.         //***For Testing in the Device***
20.         AdRequest request = new AdRequest.Builder()
21.        .AddTestDevice(AdRequest.TestDeviceSimulator)       // Simulator.
22.        .AddTestDevice("2077ef9a63d2b398840261c8221a0c9b")  // My test device.
23.        .Build();
24.  
25.         //***For Production When Submit App***
26.         //AdRequest request = new AdRequest.Builder().Build();
27.  
28.         BannerView bannerAd = new BannerView(adID, AdSize.SmartBanner, AdPosition.Top);
29.         bannerAd.LoadAd(request);
30.     }
31.  
32.     // Update is called once per frame
33.     void Update () {
34.        
35.     }
36. }

 

I think you are missing the appID.

Code (CSharp):

1. ...
2. using GoogleMobileAds.Api;
3. ...
4. public class GoogleMobileAdsDemoScript : MonoBehaviour
5. {
6.     public void Start()
7.     {
8.         #if UNITY_ANDROID
9.             string appId = "ca-app-pub-3940256099942544~3347511713";
10.         #elif UNITY_IPHONE
11.             string appId = "ca-app-pub-3940256099942544~1458002511";
12.         #else
13.             string appId = "unexpected_platform";
14.         #endif
15.  
16.         // Initialize the Google Mobile Ads SDK.
17.         MobileAds.Initialize(appId);
18.     }
19. }

https://developers.google.com/admob/unity/start