NULL REFERENCE DRVING ME INSANE!!! HELP!!!!

I am trying to make my gun rapid fire in my game. Every time I press start it throws me a Null Reference Exception here is my code. It was working fine in previous versions of unity but I decided to try the beta of 5.6 and I am now getting this error. The error is on line 26. Thanks :).

using UnityEngine; using UnityEngine.Networking;

[RequireComponent(typeof(WeaponManager))] public class PlayerShoot : NetworkBehaviour {

 private const string PLAYER_TAG = "Player";

 private PlayerWeapon currentWeapon;

 [SerializeField]
 private Camera cam;

 [SerializeField]
 private LayerMask mask;

 private WeaponManager weaponManager;

 void Start ()
 {
     if (cam == null) {
         Debug.LogError ("PlayerShoot: No camera referenced!");
         this.enabled = false;
     }

     weaponManager = GetComponent<WeaponManager> ();
 }

 void Update ()
 {
     currentWeapon = weaponManager.GetCurrentWeapon ();

     if (currentWeapon.fireRate <= 0f) {
         if (Input.GetButtonDown ("Fire1")) {
             Shoot ();
         }
     } else {
         if (Input.GetButtonDown ("Fire1")) {
             InvokeRepeating ("Shoot", 0f, 1f / currentWeapon.fireRate);
         } else if (Input.GetButtonUp ("Fire1")) {
             CancelInvoke ("Shoot");
         }
     }
 }

 [Client]
 void Shoot ()
 {
     RaycastHit _hit;
     if (Physics.Raycast (cam.transform.position, cam.transform.forward, out _hit, currentWeapon.range, mask)) {
         if (_hit.collider.tag == PLAYER_TAG) {
             CmdPlayerHit (_hit.collider.name, currentWeapon.damage);
         }
     }
 }

 [Command]
 void CmdPlayerHit (string _playerID, int _damage)
 {
     Debug.Log (_playerID + " has been shot.");

     PlayerManager _player = GameController.GetPlayer (_playerID);
     _player.RpcTakeDamage (_damage);
 }

}