NullReferenceException: Object reference not set to an instance of an object

Hello everyone, I'm making a Pong clone right now and atm I am trying to make the score. Whats supposed to happen is that when you hit the right or left wall you score a point. But this error keeps popping up:

NullReferenceException: Object reference not set to an instance of an object Ball.OnTriggerEnter2D (UnityEngine.Collider2D other) (at Assets/Scripts/Ball.cs:53)

I'm using 2 scripts, here's the first one:

using UnityEngine; using System.Collections;

public class Ball : MonoBehaviour { public float speed = 30;

 public GameManager gm;

 void Start () 
 {
     GetComponent<Rigidbody2D> ().velocity = Vector2.right * speed;

     GameObject g = GameObject.Find ("GM");
     GameManager gm = g.GetComponent<GameManager> ();
 }

 void OnCollisionEnter2D (Collision2D col)
 {
     if(col.gameObject.name == "RacketLeft")
     {
         float y = hitFactor(transform.position, col.transform.position, col.collider.bounds.size.y);

         Vector2 dir = new Vector2 (1, y).normalized;

         GetComponent<Rigidbody2D>().velocity = dir * speed;
     }

     if (col.gameObject.name == "RacketRight") 
     {
         float y = hitFactor(transform.position, col.transform.position, col.collider.bounds.size.y);

         Vector2 dir = new Vector2 (-1, y).normalized;
         
         GetComponent<Rigidbody2D>().velocity = dir * speed;
     }
 }

 float hitFactor(Vector2 ballPos, Vector2 racketPos, float racketHeight)
 {
     return(ballPos.y - racketPos.y) / racketHeight;
 }

 void OnTriggerEnter2D (Collider2D other)
 {
     if(other.gameObject.CompareTag("WallR"))
     {
         gm.countL += 1;
         gm.SetCountText();
     }

     if(other.gameObject.CompareTag("WallL"))
     {
         gm.countR += 1;
         gm.SetCountText();
     }
 }

}

Here's the second one:

using UnityEngine; using UnityEngine.UI; using System.Collections;

public class GameManager : MonoBehaviour {

 public Text rScore;
 public Text lScore;
 //public Text roundWinner;

 /*
 public GameObject ball;
 public GameObject ballSpawn;
 */

 public int countR;
 public int countL;

 void Start () 
 {
     countR = 0;
     countL = 0;
     //roundWinner.text = "";
     SetCountText ();
 }

 public void SetCountText()
 {
     rScore.text = "" + countR.ToString ();
     lScore.text = "" + countL.ToString ();
 }

 /*
 void SetCountTextL ()
 {

 }
 */

}

Thanks in advance :)