Selecting enemies with mouse button

I am trying to program my game so that i can select enemies by just clicking on them here is my script

using UnityEngine; using System.Collections;

using System.Collections.Generic;

public class Targeting : MonoBehaviour { public List targets; public Transform selectedTarget;

 private Transform myTransform;
 
     // Use this for initialization
 void Start () {
     targets = new List<Transform>();
     selectedTarget = null;
     myTransform = transform;
     
     AddAllEnemies();
 }
 
 public void AddAllEnemies()
 {
     GameObject[] go = GameObject.FindGameObjectsWithTag("Enemy");
     
     foreach(GameObject enemy in go)
         AddTarget(enemy.transform);
 }
 
 public void AddTarget(Transform enemy)
 {
     targets.Add(enemy);
 }
 
 private void SortTargetsByDistance()
 {
     targets.Sort(delegate(Transform t1, Transform t2) {
             return Vector3.Distance(t1.position, myTransform.position).CompareTo(Vector3.Distance(t2.position, myTransform.position));
                   });
 }
     private void TargetEnemy()
 {
     if(selectedTarget == null)
     {
         SortTargetsByDistance();
     selectedTarget = targets[0];
     }
     else
     {
     int index = targets.IndexOf(selectedTarget);
     
         if(index < targets.Count - 1)
         {
             index++;
         }
         else
         {
             index = 0;
         }
         DeselectTarget();
         selectedTarget = targets[index];
     }
         SelectTarget();
     }
     
     private void SelectTarget()
 {
     selectedTarget.renderer.material.color = Color.red;
     
     PlayerAttack pa = (PlayerAttack)GetComponent("PlayerAttack");
     
     pa.target = selectedTarget.gameObject;
 }
 
 private void DeselectTarget()
 {
     selectedTarget.renderer.material.color = Color.blue;
     selectedTarget = null;
 }
 
     // Update is called once per frame
 void Update () {
     
     
     //Only search if clicked
 if( Input.GetMouseButtonDown(0) ){
     //Shoot ray from mouse position
     Ray ray = Camera.mainCamera.ScreenPointToRay( Input.mousePosition );
     RaycastHit[] hits = Physics.RaycastAll( ray );
     bool gotTarget = false; //This is so we can deselect if we didn't click anything
     foreach( RaycastHit hit in hits ){ //Loop through all the hits
         if( hit.transform.gameObject.layer == 8 ) { //Make a new layer for targets
             //You hit a target!
             DeselectTarget(); //Deselect the old target
             selectedTarget = hit.transform;
             SelectTarget(); //Select the new target
             gotTarget = true; //Set that we hit something
             break; //Break out because we don't need to check anymore
         }
     }
     if( !gotTarget ) DeselectTarget(); //If we missed everything, deselect
 }

} }

Any help altering this would be appreciated i am programming C# and unity says there is and error NullReferenceException: Object reference not set to an instance of an object Targeting.DeselectTarget () (at Assets/Scripts for game/Targeting.cs:76) Targeting.Update () (at Assets/Scripts for game/Targeting.cs:100) so any help would appreciated thanks