Knockback script NullReferenceExpection Problem !

[umutxmen]

I’m writing a knockback script but has failed.
this code is gives error =

rb2d.AddForce(new Vector3(-kbDirection.x * -100, -kbDirection.y * kbPower, transform.position.z));

All codes

using UnityEngine;
    using System.Collections;
    using UnityStandardAssets.CrossPlatformInput;
    using UnityEngine.UI;
    public class Player : MonoBehaviour
    {
        public float moveForce = 5;
        public float boostMultiplier = 20;
        Rigidbody2D myBody;
    
        //State
        public int curHealth;
        public int maxHealth = 5;
    
        private Rigidbody2D rb2d;
        private Player player;
    
        private GameMaster gm;
    
        public void resetPosition()
    
        {
            transform.position = Vector3.zero;
            myBody.velocity = Vector3.zero;
        }
    
        void Start()
        {
            myBody = GetComponent<Rigidbody2D>();
            gm = GameObject.FindGameObjectWithTag("GameMaster").GetComponent<GameMaster>();
    
            curHealth = maxHealth;
        }
    
        void Update()
        {
            Vector2 moveVec = new Vector2(CrossPlatformInputManager.GetAxis("Horizontal"),
                                          CrossPlatformInputManager.GetAxis(" ")) * moveForce;
            bool isBoosting = CrossPlatformInputManager.GetButton("Boost");
            myBody.AddForce(moveVec * (isBoosting ? boostMultiplier : 1));
    
            if (CrossPlatformInputManager.GetButton("Jump"))
                transform.Translate(Vector3.up * 4 * Time.deltaTime);
    
            if (curHealth > maxHealth)
            {
                curHealth = maxHealth;
            }
    
            if (curHealth <= 0)
            {
    
                curHealth = 0;
    
                Die();
            }
    
        }
        void OnTriggerEnter2D(Collider2D col)
        {
            if (col.CompareTag("Coin"))
            {
                Destroy(col.gameObject);
                gm.points += 1;
            }
    
    
        }
    
        void Die()
        {
    
            Application.LoadLevel(Application.loadedLevel);
    
        }
    
        public void Damage(int dmg)
        {
    
            if (curHealth < dmg)
            {
                dmg = curHealth;
            }
            curHealth -= dmg;
            gameObject.GetComponent<Animation>().Play("Player_RedFlash");
        }
    
        public IEnumerator Knockback(float kbDuration, float kbPower, Vector3 kbDirection)
        {
            float timer = 0;
    
            while (kbDuration > timer)
            {
                timer += Time.deltaTime;
    
         
                    rb2d.AddForce(new Vector3(-kbDirection.x * -100, -kbDirection.y * kbPower, transform.position.z));
                
    
                yield return 0;
    
            }
    
        }
    }

[umutxmen]

Please help me !