Tree chooping

Hi guys, I need a little help! I'm creating a game for android and I want that my character chop a tree but i get this error: NullReferenceException: Object reference not set to an instance of an object MeleeSystem.AttackTree () (at Assets/Myscript/MeleeSystem.cs:43) MeleeSystem.Update () (at Assets/Myscript/MeleeSystem.cs:31)

Here there are the scripts:

 using UnityEngine;
 using System.Collections;
 using UnityStandardAssets.CrossPlatformInput;
 public class MeleeSystem : MonoBehaviour {
 
     public int minDamage = 25;
     public int maxDamage = 50;
     public float weaponRange = 3.5f;
 
     public Camera FPSCamera;
     private TreeHealt treeHealth;
 
 
     private void Update()
     {
         Ray ray = FPSCamera.ScreenPointToRay(new Vector2(Screen.width / 2, Screen.height / 2));
         RaycastHit hitInfo;
 
         Debug.DrawRay(ray.origin, ray.direction * weaponRange, Color.green);
 
         if (CrossPlatformInputManager.GetButtonDown("Action")!=null )
         {
             Debug.Log ("OK1");
             if (Physics.Raycast(ray, out hitInfo, weaponRange))
             {
                 Debug.Log ("OK2");
                 if (hitInfo.collider.tag == "Tree") // If we hit the tag "Tree"
                 {
 
                 treeHealth = hitInfo.collider.GetComponent<TreeHealt>();
                 AttackTree();
                 }
 
 
 
             }
         }
 
     }
 
     private void AttackTree()
     {
         Debug.Log("Hit Tree" + "Tree Health = " + treeHealth.health);
 
         int damage = Random.Range(minDamage, maxDamage);
         treeHealth.health -= damage;
 
     }
 
 }

Here there is another script

 using UnityEngine;
 using System.Collections;
 
 public class TreeHealt : MonoBehaviour {
     public int health =30;
     // Use this for initialization
     void Start () {
     
     }
     Rigidbody treeBody;
     // Update is called once per frame
     void Update () {
         if (health <= 0) {
             
             treeBody.isKinematic = false;
             treeBody.AddForce (transform.forward *8);
             Destroy (gameObject);
         }
     }
 }