problem with targeting system with raycast;

HI I'm new to programming and I'm trying to join a couple of scripts. the raycast part of the script seems to return an nullreferenceexception and I don't find the solution. The code is :

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 public class TargettingSystem : MonoBehaviour {
 
 
     public List <Transform> targets;
     public Transform selectedUnit;
     void Start()
     {
         targets = new List <Transform> ();
         AddAllEnemies ();
         selectedUnit = null;
     }
     void Update () 
     {
         if (Input.GetKeyDown (KeyCode.Tab)) 
         {
             targetEnemy ();
         }
         if (Input.GetMouseButtonDown(0))  
         {
             SelectTarget ();
         }
     }
     void SelectTarget()
     {
         
         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         RaycastHit hit;
         if (Physics.Raycast (ray, out hit, 10000)) 
         {
             if (hit.transform.tag == "Enemy") 
             {
                 selectedUnit = hit.transform.gameObject.transform;
             }
         }
     }
     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 targetEnemy()
     {
         selectedUnit = targets [0];
     }

}