How can I stop getting Null Reference Exception?

I am making a FPS, but on my AI, I get a Null Reference Exception. I would like to make the script check if the tagged player exists, but I can't figure it out. Could someone please point me in the right direction? Here is the code:

 var Distance;
 var Target : Transform;
 var lookAtDistance = 250;
 //var attackRange = 15.0;
 var moveSpeed = 5.0;
 var Damping = 6.0;
 //Prefabs for different hits.
 var FleshPrefab : Transform;
 var DirtPrefab : Transform;
 var MetalPrefab : Transform;
 var ConcretePrefab : Transform;
 var WoodPrefab : Transform;
 //Bullet holes
     public var woodBulletHole : GameObject[];
     //End of prefabs for differen hits.
     var gunshotSound : AudioClip;
     private var accuracy : float = 2.5;
     var TheDammage : int = 10;
     var attackRepeatTime = 3.5;
     var Health = 100;
     private var attackTime : float;
     var PlayerHealth : PlayerHealth;
     
     //Ragdoll and AI death
     var DeathRagdoll : GameObject;
     var AI : Transform;
     
     function Update ()
     {
         if (Target == null)
         {
             Target = GameObject.FindWithTag("Player").transform;
         }
         if (Target)
         {
             Distance = Vector3.Distance(Target.position, transform.position);
             
             if (Distance < lookAtDistance)
             {
                 renderer.material.color = Color.yellow;
                 lookAt();
             }
             
             if (Distance > lookAtDistance)
             {
                 renderer.material.color = Color.green;
             }
             
             /*if (Distance < attackRange)
             {
                 renderer.material.color = Color.red;
                 attack ();
             }*/
         }
     }
     
     function lookAt ()
     {    
         var rotation = Quaternion.LookRotation(Target.position - transform.position);
         transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * Damping);
         attackRepeatTime = Random.Range(3.5,4.5);
         accuracy = Random.Range(0.01, 12.5);
         if (PlayerHealth.health >= 0)
         {
             Fire();
         }
     }
     
     function attack ()
     {
         transform.Translate(Vector3.forward * moveSpeed * Time.deltaTime);
     }
     
     function Fire()
     {
         if (Time.time > attackTime)
         {
             AudioSource.PlayClipAtPoint(gunshotSound, transform.position, 10000);
             var hit : RaycastHit;
             var BulletDirection = transform.TransformDirection(new Vector3(Random.Range(-accuracy, accuracy) * 0.002, Random.Range(-accuracy, accuracy) *0.001,1));
             if (Physics.Raycast(transform.position, BulletDirection, hit, Mathf.Infinity))
             {
                 if (hit.transform.gameObject.tag=="Player")
                 {
                     hit.transform.SendMessage("Damage", TheDammage, SendMessageOptions.DontRequireReceiver);
                     var FleshClone = Instantiate(FleshPrefab, hit.point, transform.rotation);
                     Destroy(FleshClone.gameObject, 2);
                 }
                 if (hit.transform.gameObject.tag=="concrete")
                 {
                     var ConcreteClone = Instantiate(ConcretePrefab, hit.point, transform.rotation);
                     Destroy(ConcreteClone.gameObject, 2);
                 }
                 if (hit.transform.gameObject.tag=="dirt")
                 {
                     var DirtClone = Instantiate(DirtPrefab, hit.point, transform.rotation);
                     Destroy(DirtClone.gameObject, 2);
                 }
                 if (hit.transform.gameObject.tag=="metal")
                 {
                     var MetalClone = Instantiate(MetalPrefab, hit.point, transform.rotation);
                     Destroy(MetalClone.gameObject, 2);
                 }
                 if (hit.transform.gameObject.tag=="wood")
                 {
                     var WoodClone = Instantiate(WoodPrefab, hit.point, transform.rotation);
                     Destroy(WoodClone.gameObject, 2);
                     Instantiate(woodBulletHole[Random.Range(0,3)], hit.point, Quaternion.FromToRotation(Vector3.up, hit.normal));
                 
                 }
             }
             attackTime = Time.time + attackRepeatTime;
         }
         
     }
     
     function ApplyDammage (TheDammage : int)
     {
         Health -= TheDammage;
         
         if(Health <= 0)
         {
             Dead();
         }
     }
     
     function Dead()
     {
         Instantiate (DeathRagdoll , AI.position, transform.rotation);
         Destroy (gameObject);
     }