can someone help me with my code error

I am just starting out making a game and all is going well exept for my defense of the player for some reason. it shows up as a

NullReferenceException: Object reference not set to an instance of an object hurtplayer.OnCollisionEnter2D (UnityEngine.Collision2D other) (at Assets/script/hurtplayer.cs:20)

can someone please help me find out were I cant seam to find the problem.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class playerstats : MonoBehaviour {
     public int currentlvl;
     public int exp;
     public int[] tolvlup;
     public int[] hplevels;
     public int[] attacklevels;
     public int[] defenselevels;
     public int chealth;
     public int cattack;
     public int cdeffense;
     private playerhealth playerhealth1;
 
 
 
     void Start () {
         chealth = hplevels [1];
         cattack = attacklevels [1];
         cdeffense = defenselevels [1];
         playerhealth1 = FindObjectOfType <playerhealth> ();
 
 
     }
     
 
     void Update () {
         if (exp >= tolvlup [currentlvl]) {
             //currentlvl++;
             levelup ();
 
         }
 
 
     }
     public void addexp(int expadd){
         exp += expadd;
 
     }
     public void levelup (){
         currentlvl++;
         chealth = hplevels [currentlvl];
         playerhealth1.playermhealth = chealth;
         playerhealth1.playerchealth += chealth - hplevels[currentlvl - 1];
         cattack = attacklevels [currentlvl];
         cdeffense = defenselevels [currentlvl];
 
 
 
     }
 
 }
 

 using UnityEngine;
 using System.Collections;
 public class hurtplayer : MonoBehaviour{
     
 
     public int damagetogive;
     private int currentdamage;
     private playerstats ps;
 
 
     void start(){
         ps = FindObjectOfType<playerstats> ();
 
     }
     void update(){
 
     }
     void OnCollisionEnter2D(Collision2D other){
         if (other.gameObject.tag == "Player") {
             
             currentdamage = damagetogive - ps.cdeffense;
             //currentdamage = damagetogive;           
 
 
             other.gameObject.GetComponent<playerhealth> ().hurtplayer (currentdamage);
         }
 
 
     }
 
 
 }

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class playerhealth : MonoBehaviour {
     public int playermhealth;
     public int playerchealth;
 
     // Use this for initialization
     void Start () {
         playerchealth = playermhealth;
     }
     
     // Update is called once per frame
     void Update () {
         if (playerchealth < 0) {
             gameObject.SetActive (false);
 
         }
 
 
 
 
         if (playerchealth > playermhealth) {
             playerchealth = playermhealth;
         }
 
 
 
 
         
     }
     public void hurtplayer(int currentdamage)
     {
 
         playerchealth -= currentdamage;
     }
 
     public void setmaxhealth()
     {
         playermhealth = playerchealth;
     }
 
 
 }