Unity IOS : Game crash in Facebook audience network while showing ad

I have integrated Facebook audience network for showing ads in my game.

I show Interstitial Ads. Showing ads in ANDROID platform Successfully done but when i run my game on IOS it's getting crash while showing ads.

-My script for load and show Interstitial ads

using UnityEngine;
using AudienceNetwork;
using AudienceNetwork.Utility;

public class InterstitialAdScene : MonoBehaviour
{
    public static InterstitialAdScene instance;

    private InterstitialAd interstitialAd;
    private bool isLoaded;
    #pragma warning disable 0414
    private bool didClose;
    #pragma warning restore 0414

private void Awake()
{
    instance = this;

    AudienceNetworkAds.Initialize();

  //  AdSettings.AddTestDevice("38dab032-c36a-44e9-a7ed-f39eeef3093e");
    if (!isLoaded)
    {
        LoadInterstitial();
    }
}

// Load button
public void LoadInterstitial()
{
    Debug.Log("Loading interstitial ad...");

    // Create the interstitial unit with a placement ID (generate your own on the Facebook app settings).
    // Use different ID for each ad placement in your app.
#if UNITY_ANDROID
    interstitialAd = new InterstitialAd("917812518650022_917813945316546");
#endif

#if UNITY_IOS
    interstitialAd = new InterstitialAd("377269886512141_377270299845433");
#endif
    interstitialAd.Register(gameObject);

    // Set delegates to get notified on changes or when the user interacts with the ad.
    interstitialAd.InterstitialAdDidLoad = delegate ()
    {
        Debug.Log("Interstitial ad loaded.");
        isLoaded = true;
        didClose = false;
        string isAdValid = interstitialAd.IsValid() ? "valid" : "invalid";
    };
    interstitialAd.InterstitialAdDidFailWithError = delegate (string error)
    {
      //  Debug.Log("Interstitial ad failed to load with error: " + error);
    };
    interstitialAd.InterstitialAdWillLogImpression = delegate ()
    {
       // Debug.Log("Interstitial ad logged impression.");
    };
    interstitialAd.InterstitialAdDidClick = delegate ()
    {
       // Debug.Log("Interstitial ad clicked.");
    };
    interstitialAd.InterstitialAdDidClose = delegate ()
    {
       // Debug.Log("Interstitial ad did close.");
        didClose = true;
        if (interstitialAd != null)
        {
            interstitialAd.Dispose();
        }
    };

    // Initiate the request to load the ad.
    interstitialAd.LoadAd();
}

// Show button
public void ShowInterstitial()
{
    if (isLoaded)
    {
        interstitialAd.Show();
        isLoaded = false;
    }
    else
    {
        Debug.Log("Ad not loaded. Click load to request an ad.");
    }
}

void OnDestroy()
{
    // Dispose of interstitial ad when the scene is destroyed
    if (interstitialAd != null)
    {
        interstitialAd.Dispose();
    }
    Debug.Log("InterstitialAdTest was destroyed!");
}
}

-getting this crash while showing ad:

Terminating app due to uncaught exception 'FBFinalClassViolationException', reason: 'FBInterstitialAd is a final class and cannot be subclassed. FBInterstitialAd'

Please help me for this i don't know how to fixed it.

Thanks