Admob Native Ad convert Issue

I am using google Admob Native Ad to target specific locations in a game. While following the steps, I encountered the following error:

 AdMobManager.cs(37,30): error CS1503: Argument 1: cannot convert from 'string' to 'System.Action<GoogleMobileAds.Api.InitializationStatus>'

The following is where the error is occurring on Mobile.Initialize:

 private string idApp, idNative;

// Start is called before the first frame update
void Start(){
    idApp = "ca-app-pub-3940256099942544~3347511713";
    idNative = "ca-app-pub-3940256099942544/2247696110";

   MobileAds.Initialize(idApp);
   RequestNativeAd();
}

UPDATE: I did the following which resolved the issue. However, I am unable to view the native ad. I am getting a null reference error:

NullReferenceException: Object reference not set to an instance of an object

It is referring to the following lines that is in the update method:

Texture2D iconTexture = this.adNative.GetIconTexture();
Texture2D iconAdChoices = this.adNative.GetAdChoicesLogoTexture();
string headline = this.adNative.GetHeadlineText();
string cta = this.adNative.GetCallToActionText();
string advertiser = this.adNative.GetAdvertiserText();
adIcon.texture = iconTexture;
adChoices.texture = iconAdChoices;
adHeadline.text = headline;
adAdvertiser.text = advertiser;
adCallToAction.text = cta;

The following images are what I did in Unity:

Image1

Image2

The following is the code where I am render the Native Ads:

using System.Diagnostics;
using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using UnityEngine.Events;
using System.Threading.Tasks;
using Debug = UnityEngine.Debug;
using GoogleMobileAds.Api;
using GoogleMobileAds.Common;

public class AdMobManager : MonoBehaviour
{
    private NativeAd adNative;
    private bool nativeLoaded = false;

    private string idApp, idNative;

    [SerializeField] GameObject adNativePanel;
    [SerializeField] RawImage adIcon;
    [SerializeField] RawImage adChoices;
    [SerializeField] Text adHeadline;
    [SerializeField] Text adCallToAction;
    [SerializeField] Text adAdvertiser;

    // Start is called before the first frame update
    void Start(){
        idApp = "ca-app-pub-3940256099942544/6300978111";
        idNative = "ca-app-pub-3940256099942544/2247696110";

        List<String> deviceIds = new List<String>() { AdRequest.TestDeviceSimulator };

        // Add some test device IDs (replace with your own device IDs).
        #if UNITY_ANDROID
                deviceIds.Add(idApp);
        #endif


       MobileAds.Initialize(HandleInitCompleteAction);
    }

    void Update(){
        Texture2D iconTexture = this.adNative.GetIconTexture();
        Texture2D iconAdChoices = this.adNative.GetAdChoicesLogoTexture();
        string headline = this.adNative.GetHeadlineText();
        string cta = this.adNative.GetCallToActionText();
        string advertiser = this.adNative.GetAdvertiserText();
        adIcon.texture = iconTexture;
        adChoices.texture = iconAdChoices;
        adHeadline.text = headline;
        adAdvertiser.text = advertiser;
        adCallToAction.text = cta;

        //register gameobjects
        adNative.RegisterIconImageGameObject(adIcon.gameObject);
        adNative.RegisterAdChoicesLogoGameObject(adChoices.gameObject);
        adNative.RegisterHeadlineTextGameObject(adHeadline.gameObject);
        adNative.RegisterCallToActionGameObject(adCallToAction.gameObject);
        adNative.RegisterAdvertiserTextGameObject(adAdvertiser.gameObject);

        adNativePanel.SetActive(true); //show ad panel
    }

    private void HandleInitCompleteAction(InitializationStatus initstatus) {
        Debug.Log("Initialization complete.");

        // Callbacks from GoogleMobileAds are not guaranteed to be called on
        // the main thread.
        // In this example we use MobileAdsEventExecutor to schedule these calls on
        // the next Update() loop.
        MobileAdsEventExecutor.ExecuteInUpdate(() => {
            RequestNativeAd();
        });
    }

    private void RequestNativeAd(){
        AdLoader adLoader = new AdLoader.Builder (idNative).ForNativeAd().Build();
        adLoader.OnNativeAdLoaded += this.HandleOnUnifiedNativeAdLoaded;
        adLoader.LoadAd(AdRequestBuild());
    }

    //events
    private void HandleOnUnifiedNativeAdLoaded (object sender, NativeAdEventArgs args){
        this.adNative = args.nativeAd;
    }

    AdRequest AdRequestBuild(){
        return new AdRequest.Builder().Build();
    }
}