NullReferenceException: Object reference not set to an instance of an object Player.OnCollisionEnter2D

Question

NullReferenceException: Object reference not set to an instance of an object Player.OnCollisionEnter2D (UnityEngine.Collision2D other) (at Assets/Scripts/Player.cs:93)

This warning appears in console every time my player collide an enemy. How can I solve that?

line 93 is StartCoroutine(player.Knockback(0.002f, 100, player.transform.position));

using System.Collections.Generic;
using UnityEngine;

public class Player : MonoBehaviour
{
    public GameObject Death;
    public int maxHealth = 100;
    public int currentHealth;
    public Animator animator;
    private Rigidbody2D rb2d;
    public HealthBar healthBar;
    public float respawnDelay;  
    private Player player;
    public GameObject coins;
    public GameObject coinado;
    public GameObject healthBAR;
    Vector3 pushDirection;
    Rigidbody2D rb;


    // Start is called before the first frame update
    void Start()
    {
        currentHealth = maxHealth;
        healthBar.SetMaxHealth(maxHealth);
    }

    void Update()
    {
        /*if (Input.GetKeyDown(KeyCode.Space))
        {
            TakeDamage(20);
        }*/
        if(currentHealth > maxHealth) {
            currentHealth = maxHealth;
        }

        if(currentHealth <= 0) {
            currentHealth = 0;
            GameObject.Find("Player").GetComponent<CharacterController2D>().enabled = false;
            GameObject.Find("Player").GetComponent<PlayerMovement>().enabled = false;
            Die();
        }

    }

    public void TakeDamage(int damage)
    {
        currentHealth -= damage;
        FindObjectOfType<Audiio>().Play("PlayerHurt");
        animator.SetTrigger("Hurt");
        if (currentHealth <=0)
        {
            Die();
        }

        healthBar.SetHealth(currentHealth);
    }
    void Die() {
        Death.SetActive(true);
        coins.SetActive(false);
        coinado.SetActive(false);
        healthBAR.SetActive(false);
        Time.timeScale = 0.5f;
        FindObjectOfType<Audiio>().Play("PlayerDeath");
        animator.SetBool("IsDead", true);
        Respawn();

    }
    public void Respawn(){
        Time.timeScale = 1f;
        StartCoroutine("RespawnCoroutine");
    }
    public IEnumerator RespawnCoroutine(){
        yield return new WaitForSeconds(respawnDelay);
        Application.LoadLevel(Application.loadedLevel);
    }
    public IEnumerator Knockback(float knockDur, float knockbackPwr, Vector3 knockbackDir) {
        float timer = 0;
        while (knockDur > timer) {
            timer +=Time.deltaTime;
            //rb2d.AddForce(new Vector3(knockbackDir.x *-2000, knockbackDir.y*knockbackPwr, transform.position.z));
        }
        yield return 0;
    }

    private void OnCollisionEnter2D(Collision2D other)
    {
        if(other.gameObject.tag == "Enemy")
        {
            TakeDamage(30);
            StartCoroutine(player.Knockback(0.002f, 100, player.transform.position));

        }
    }
}