I want to put my admob in my code

Questions & Answers
1

I got error like this
NullReferenceException: Object reference not set to an instance of an object
BirdMovement.Update () (at Assets/BirdMovement.cs:51)

void Update() {
		if(dead) {
			deathCooldown -= Time.deltaTime;
     ***this is line 51  ---- > interstitial.Show();***
			if(deathCooldown <= 0) {
				if(Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0) ) {
					Application.LoadLevel( Application.loadedLevel );
				}
			}

		}
		else {
			if(Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0) ) {
				didFlap = true;
			}
		}

	}
2

Do you think this is a good place but it not show up any solution

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

public class BirdMovement : MonoBehaviour {

	Vector3 velocity = Vector3.zero;
	public float flapSpeed    = 100f;
	public float forwardSpeed = 1f;

	bool didFlap = false;

	Animator animator;

	public bool dead = false;
	float deathCooldown;

	public bool godMode = false;

	private InterstitialAd interstitial;

	// Use this for initialization

	void Start () {

		animator = transform.GetComponentInChildren<Animator>();

		if(animator == null) {
			Debug.LogError("Didn't find animator!");
		}
	}

	
	private void RequestInterstitial()
	{
		string adUnitId = "my admob id";
		// Initialize an InterstitialAd.
		InterstitialAd interstitial = new InterstitialAd(adUnitId);
		// Create an empty ad request.
		AdRequest request = new AdRequest.Builder().Build();
		// Load the interstitial with the request.
		interstitial.LoadAd(request);
		
		if (interstitial.IsLoaded()) {
			interstitial.Show();
		}
	}

	// Do Graphic & Input updates here
	void Update() {
		if(dead) {
			deathCooldown -= Time.deltaTime;

			if(deathCooldown <= 0) {
				if(Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0) ) {
					Application.LoadLevel( Application.loadedLevel );
				}
			}
		}
		else {
			if(Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0) ) {
				didFlap = true;
			}
		}
	}

	
	// Do physics engine updates here
	void FixedUpdate () {

		if(dead)
			return;

		GetComponent<Rigidbody2D>().AddForce( Vector2.right * forwardSpeed );

		if(didFlap) {
			GetComponent<Rigidbody2D>().AddForce( Vector2.up * flapSpeed );
			animator.SetTrigger("DoFlap");


			didFlap = false;
		}

		if(GetComponent<Rigidbody2D>().velocity.y > 0) {
			transform.rotation = Quaternion.Euler(0, 0, 0);
		}
		else {
			float angle = Mathf.Lerp (0, -90, (-GetComponent<Rigidbody2D>().velocity.y / 3f) );
			transform.rotation = Quaternion.Euler(0, 0, angle);
		}
	}

	void OnCollisionEnter2D(Collision2D collision) 
		{
		if(godMode)
			return;

		animator.SetTrigger("Death");
		dead = true;
		deathCooldown = 0.5f;
		}

}