Problem with clicking to target enemies

So I'm looking to make an isometric game where the player clicks on enemies to target them, and the player will then move towards the enemy and begin attacking.

The player object has the following script attached

 using UnityEngine;
 using System.Collections;
 
 public class playerControllerScript : MonoBehaviour {
 
     public Transform Target;
     public Transform myTransform;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if(!Target)
         {
             Debug.Log("No target");
         }
         else
         {
             myTransform = transform;
             Debug.DrawLine(transform.position, Target.position);
         }
     }
     
     public void changeTarget(Transform newTarget)
     {
         Target = newTarget;
     }
 }

so that the target is a position the player will move to, rather than an object (I thought this would be easier, all enemies will be spawned when the player gets close enough to the spawn point, and all enemies will have the same targetting script). This is it in the stages of sorting out targetting, attacking and moving will get done soon.

and the enemies have the following script attached

 using UnityEngine;
 using System.Collections;
 
 public class targettingScript : MonoBehaviour {
 
     public Transform myTransform;
 
     // Use this for initialization
     void Start () {
         
     }
     
     // Update is called once per frame
     void Update () {
         myTransform = transform;
     }
     
     void OnMouseDown()
     {
         playerControllerScript player = GetComponent<playerControllerScript>();
         player.changeTarget(myTransform);
         Debug.Log("Clicked on");
     }
 }

So when an enemy is clicked on, the player will have a new transform to move towards and attack. Again, this is still in the stages of sorting out targetting.

But when I click on a target, I get "NullReferenceException: Object reference not set to an instance of an object targettingScript.OnMouseDown ()" and nothing happens beyond that.

How do I fix this? Thanks in advance.