Public void has problem while activing from another class.

Greetings! I want to active public void EnemyDestroyed in another class, Enemy class; but once this function is activated, in the console Unity says: NullReferenceException: Object reference not set to an instance of an object Enemy.Update () (at Assets/Scripts/Enemy.cs:51). In the inspector of enemyPrefab, where the Enemy script is attached to, there is a blank instance called Spawn Enemy, and there is no such thing that can be filled in that blank. In the script, monodevelope does consider SpawnEnenmy in Enemy script as a public class, but in the inspector it doesn't. I guess I did something wrong while calling the public void, but just couldn't figure it out by myself.

public class SpawnEnemy : MonoBehaviour {

 public Rigidbody enemy1;
 public Rigidbody enemy2;
 public Rigidbody enemy3;
 public GameObject SpawnPoint;
 public int enemyNum = 0;
 public int difficulty = 0;

 private float E = 0;

 void Refill ()
 {
     E += 1.0f * Time.deltaTime;
 }

 public void EnemyDestroyed ()
 {
     enemyNum -= 1;
     difficulty += 1;
 }

 // Update is called once per frame
 void Update () 
 {
     if (enemyNum < 7 && difficulty <= 10 && E >= 1)
     {
         Rigidbody enemy1Instance;
         enemy1Instance = Instantiate (enemy1, SpawnPoint.transform.position, SpawnPoint.transform.rotation) as Rigidbody;
         enemy1Instance.AddForce (SpawnPoint.transform.forward * 1);

         enemyNum +=1;
         E -= 3.0f;
     }

     if (enemyNum <= 5 && difficulty <= 30 && E >= 1 && difficulty > 10)
     {
         Rigidbody enemy2Instance;
         enemy2Instance = Instantiate (enemy2, SpawnPoint.transform.position, SpawnPoint.transform.rotation) as Rigidbody;
         enemy2Instance.AddForce (SpawnPoint.transform.forward * 1);

         enemyNum +=1;
         E -= 3.0f;
     }

     if (enemyNum <= 4 && difficulty <= 60 && E >= 1 && difficulty > 30)
     {
         Rigidbody enemy3Instance;
         enemy3Instance = Instantiate (enemy3, SpawnPoint.transform.position, SpawnPoint.transform.rotation) as Rigidbody;
         enemy3Instance.AddForce (SpawnPoint.transform.forward * 1);

         enemyNum +=1;
         E -= 3.0f;
     }

     if (E <= 3)
     {
         Refill ();
     }
 }

}

public class Enemy : MonoBehaviour {

 public SpawnEnemy Spawn;
 public int life = 200;
 public int moveSpeed = 1;

 private bool move;
 private int[] damage = {50, 25, 30};

 void Start ()
 {
     move = true;
 }

 void OnTriggerEnter (Collider other)
 {
     if (other.gameObject.CompareTag ("CannonBall")) 
     {
         life -= damage[0];
     }
     
     if (other.gameObject.CompareTag ("FireBall")) 
     {
         life -= damage[1];
     }
     
     if (other.gameObject.CompareTag ("IceBall")) 
     {
         life -= damage[2];
     }
     
     if (other.gameObject.CompareTag ("Gate"))
     {
         move = false;
     }
 }

 // Update is called once per frame
 void Update () 
 {    
     if (move == true)
     {
         transform.Translate(Vector3.forward * moveSpeed * Time.deltaTime);
     }

     if (life <= 0)
     {
         Spawn.EnemyDestroyed ();
     }
 }

 void LateUpdate ()
 {
     if (life <= 0)
     {
         Destroy (gameObject);
     }
 }

}