Adding instances of objects into generic list

Hello guys, I am working on simple tower defense game and I have a problem with adding GameObjects I instantiated to list. Simply, when I add instance to a list, another instance will not be instantiated...

This code is for spawning enemies and adding them to list.

 using UnityEngine;
 using System.Collections;
 
 public class GameController : MonoBehaviour {
 
     public Transform spawnPoint;
     public GameObject enemy;
     public GameObject tower;
     
     private Ray ray;
     private RaycastHit hit;
 
     // Use this for initialization
     void Start () {
 
         StartCoroutine(SpawnMinions());
     }
     
     // Update is called once per frame
     void Update () {
 
         if(Input.GetButtonDown("Fire1")) {
             ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
             if(Physics.Raycast(ray.origin, ray.direction * 100f, out hit)) {
                 
                 Instantiate(tower, hit.point + Vector3.up * 0.5f, Quaternion.identity);
             }
         }
     }
 
     IEnumerator SpawnMinions() {
         
         while(true) {
             for(int i = 0; i < 5; i++) {
     
                 GameObject enemyInstance = Instantiate(enemy, spawnPoint.position, transform.rotation) as GameObject;
                 yield return new WaitForSeconds(0.5f);
                 Tower.enemies.Add(enemyInstance); // when I remove this line of code, it instantiates 5 enemies. But with it, it instantiates only 1... It behaves like return command in function.
             }
             yield return new WaitForSeconds(3f);
         }
     }
 }
 

And then here is Tower script... Code should check, if there is at least 1 enemy in list and if it is and is in specific range from tower, then start shoot enemy until it is destroyed:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Tower : MonoBehaviour {
 
     public GameObject bullet;
     public static List<GameObject> enemies;
     public int time;
 
     void Start() {
 
     }
 
     void Update() {
 
         time = (int)(Time.time);
 
         if(enemies.Count > 0) {
             for(int i = 0; i < enemies.Count; i++) {
             
                 while(enemies[i].gameObject != null) { // while current object exists
 
                     if(time % 2 == 0 && (Vector3.Distance(transform.position, enemies[i].transform.position) < 2)) {
 
                         Instantiate(bullet, transform.position, transform.rotation);
                         bullet.SendMessage("Fire", enemies[i].transform.position - transform.position);
                     }
                     
                     if(Vector3.Distance(transform.position, enemies[i].transform.position) > 2) break;       
                 }
             }
         }
     }
 }

And really simple bullet script:

 using UnityEngine;
 using System.Collections;
 
 public class Bullet : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     public void Fire (Transform target) {
 
         rigidbody.AddForce(target.position * 1.5f * Time.deltaTime);
     }
 }

Unity writes this error, when I start the game... Really don't know what does it mean:

NullReferenceException: Object reference not set to an instance of an object GameController+c__Iterator0.MoveNext () (at Assets/Scripts/GameController.cs:39)

And when I instantiate tower, it keeps writing:

NullReferenceException: Object reference not set to an instance of an object Tower.Update () (at Assets/Scripts/Tower.cs:19)

(Saying, that error is on this line of code: if(enemies.Count > 0))