Missile Target by using string and if target if null keep search for new target

when the missile target the enemy by using target = Getcomponent.findGameobjectwithtag<"Enemy">(). transform; is working good but when the enemy is die it giving me null reference, but i add target = get-component function to fixed update it seem to work good but it have this weird look rotation and it keep choosing new target steady. i look all over the internet and try code but not working. so i want to know if you can help me out with a new script.
i want a array of enemy. and store this array in a string right which is findgameobjectwithstag. but if the is die i, and same time i shoot a another missile how can i tell the missile to look for a another target or if target is not found move upward on the y axis and get destroy by the boundary.
just making a normal shmup nobody want to help me. seem like heat seeking missile carry me to my grave working on it for week now.please let this be the final i am tired of hurting my head.

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class MissileMove : MonoBehaviour
5. {
6.  
7.     Transform target;
8.     public int speed;
9.     public int _dmg = 100;
10.     private Health _Health;
11.  
12.  
13.     void Awake()
14.     {
15.        
16.             target = GameObject.FindGameObjectWithTag ("Enemy").transform;
17.  
18.     }
19.  
20.     void FixedUpdate()
21.     {
22.        
23.             Vector3 dir = target.position - transform.position;
24.             float angle = Mathf.Atan2 (dir.y, dir.x) * Mathf.Rad2Deg - 90;
25.             transform.rotation = Quaternion.AngleAxis (angle, Vector3.forward);
26.             transform.position = Vector3.MoveTowards (transform.position, target.position, speed * Time.deltaTime);
27.  
28.     }
29.  
30.        
31.       //  transform.Rotate(0, 0, 1.5f);
32.     void OnTriggerEnter (Collider other)
33.     {
34.         if (other.gameObject.tag == "Enemy") {
35.  
36.             _Health = other.gameObject.GetComponent<Health> ();
37.             if (_Health != null) {
38.                 _Health._Dmg (_dmg);
39.             }
40.             //other.gameObject.GetComponent<Health> ()._Dmg (_dmg);
41.             Destroy (gameObject);
42.  
43.         }
44.     }
45.        
46. }
47.  

 

Well, I would say check if the enemy is null if it is then check for another enemy if there is none then do some nice curve.

 

If there's no enemy's on screen, then it can't check for an active GO with the tag enemy, you might wan't to use a <list> then use conditions to check if there's any during runtime,

Code (CSharp):

1. List<Enemies> enemies = new list<Enemies>();
2.  
3. Update()
4. {
5. enemies = GameObject.FindGameObjectsWithTag("Enemy");
6.  
7. if (enemies.count > 0)
8. {
9. // allow detection of the transform;
10. }
11. else
12. {
13. // don't run
14. }
15. }
16.  
17.  

EDIT:

Forgot to mention you need to remove enemies from the list when they are destroyed using

Code (CSharp):

1. enemies.Remove(GONAME);

 

so if the enemy.count >0 ) the missile when follow the enemy.
and else tell the missile to do a curve or move forward to the screen
and where i put enemies.Remove(GoName). and what is go name