NullReferenceException: Object reference not set to an instance of an object
PlayerScript.OnCollisionEnter2D (UnityEngine.Collision2D col) (at Assets/Scripts/PlayerScript.cs:13)

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Enemy4 : MonoBehaviour
{

    public Transform waypoints;
    public float speed;
    private Transform waypoint;
    private int waypointIndex = -1;
    Rigidbody rb;

    void Start()
    {
        NextWaypoint();
    }
    void Awake()
    {
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {

        Vector3 dir = waypoint.transform.position - transform.position;
        dir.y = 0;

        float _speed = Time.deltaTime * speed;
        transform.Translate(dir.normalized * _speed);

        if (dir.magnitude <= _speed)
            NextWaypoint();

    }
    void NextWaypoint()
    {

        waypointIndex++;
        if (waypointIndex >= waypoints.childCount)
        {
            Destroy(gameObject);
            return;
        }
        waypoint = waypoints.GetChild(waypointIndex);

    }
}

void FixedUpdate()
    {
        Quaternion rotationY = Quaternion.AngleAxis(1, Vector3.up);
        transform.rotation *= rotationY;
    }

        leftSX = Gamepad.current.leftStick.x.ReadValue();
        leftSY = Gamepad.current.leftStick.y.ReadValue();
        rightSX = Gamepad.current.rightStick.x.ReadValue();
        rightSY = Gamepad.current.rightStick.y.ReadValue();
        print("First Axis x" + leftSX + "y:"+ leftSY);
        print("Two Axis x" + rightSX + "y:" + rightSY);

var _b = GameObject.Instantiate(_build, new Vector3(171.50f,3.76f,38.08f), new Quaternion(0,0,0,1));

   void Start () {
        
        if (isServer)
            direction = transform.rotation;
        else
            transform.rotation = direction;

        Rigidbody body = GetComponent<Rigidbody>();
        body.constraints = RigidbodyConstraints.FreezeAll;
        body.useGravity = false;

        if (!active)
        {
            if (GetComponent<MeshCollider>() != null)
                GetComponent<MeshCollider>().convex = true;
            GetComponent<Collider>().isTrigger = true;
        }
        else
        {
            body.isKinematic = true;
            foreach (Transform child in transform)
            {
                child.gameObject.SetActive(true);
            }
        }       
    }

    void OnTriggerEnter(Collider other)
    {
  if(!active)
  if(other.tag != "Terrain")
  {
   collide ++;
  }
    }
   
    void OnTriggerExit(Collider other)
    {
  if(!active)
  if(other.tag != "Terrain")
  {
   collide --;
  }
    }