Unity Google Admob Reklam Ekleme Hatası

private void Start()
{

//Hata veren kod satırı
rewardedAd = RewardBasedVideoAd.Instance;
//Hata veren kod satırı

RequestRewardedAd();

this.rewardedAd = RewardBasedVideoAd.Instance;
.
.
.

}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using GoogleMobileAds.Api;

public class RewardVideos : MonoBehaviour

{
private RewardBasedVideoAd rewardBasedVideo;


public void Start()
{
#if UNITY_ANDROID
string appId = "APPYOURID";
#elif UNITY_IPHONE
string appId = "";
#else
string appId = "unexpected_platform";
#endif

// Initialize the Google Mobile Ads SDK.
MobileAds.Initialize(appId);

// Get singleton reward based video ad reference.
this.rewardBasedVideo = RewardBasedVideoAd.Instance;
this.RequestRewardBasedVideo();
}

private void RequestRewardBasedVideo()
{
#if UNITY_ANDROID
string adUnitId = "REWARD ID niz";
#elif UNITY_IPHONE
string adUnitId = "YOUR ADS ID";
#else
string adUnitId = "unexpected_platform";
#endif

// Create an empty ad request.
AdRequest request = new AdRequest.Builder().Build();
// Load the rewarded video ad with the request.
this.rewardBasedVideo.LoadAd(request, adUnitId);
}

public void ReklamOdullu()
{
if (rewardBasedVideo.IsLoaded())
{
rewardBasedVideo.Show();
}
}
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.google.unity.ads" android:versionName="1.0" android:versionCode="1">
<uses-sdk android:minSdkVersion="14" android:targetSdkVersion="19" />
<application>
<uses-library android:required="false" android:name="org.apache.http.legacy" />
<!-- Your AdMob App ID will look similar to this sample ID: ca-app-pub-3940256099942544~3347511713 -->
</application>
</manifest>