'GameObject' has been destroyed woe...

[Apriyanto]

‘GameObject’ has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object.

when i killed one enemy and continue hit another enemy there have some message**

MissingReferenceException: The object of type 'GameObject' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
PlayerAttack.Attack () (at Assets/Garudayana Projects/Scripts/PlayerAttack.js:70)
PlayerAttack.Update () (at Assets/Garudayana Projects/Scripts/PlayerAttack.js:53)

I don’t know what happen in here , because i already make gameObject != null ;**

The first enemy i just killed already Destroy, but when i am try to hit another enemies the player still can attack and the 3D model another enemies still appears, but the notification tell, the game object are destroy .**

[aldonaletto]

gameObject alone refers to the object to which the script is attached. You should check targets instead:
function Attack()
{
for (var i = 0; i < targets.Length; i++)
{
if (targets == null) continue; // skip destroyed targets
var distance : float = Vector3.Distance(targets*.transform.position, transform.position);*
var dir : Vector3 = (targets*.transform.position - transform.position).normalized;*
…