Not sure why i'm getting NullReferenceException

Okay so here is my code and for some reason it's working the way I intended yet still returns an error :/ Can anyone please explain how to fix this?

Code:

     using UnityEngine;
     using System.Collections;
     
     public class Player : MonoBehaviour {
     
         public int playerHealth = 20;
     
         void Start () 
         {
             
         }
         
         void Update () 
         {
             if(playerHealth <= 0)
             {
                 Debug.Log("player died");
             }
         }
     
         void OnCollisionEnter2D(Collision2D other)
         {
             if (other.gameObject.tag == "Enemy")
             {
                 playerHealth -= other.gameObject.GetComponent<ChargerAI>().attackDamage;
             }
         }
     }

Error:

     NullReferenceException: Object reference not set to an instance of an object
     Player.OnCollisionEnter2D (UnityEngine.Collision2D other) (at Assets/Scripts/Player.cs:25)