Object Reference error when no object is being referenced.

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

public class Collectables : MonoBehaviour {

 private Laser laser;
 private Player player;
 private Leveling level;
 public GameObject enemy;

 private void Start()
 {
     level = GetComponent<Leveling>();
     laser = GetComponent<Laser>();
     player = GetComponent<Player>();
 }

 private void OnTriggerEnter2D(Collider2D collision)
 {
     if (collision.tag == "Player")
     {
         Instantiate(enemy, new Vector2(Random.Range(-2, 3), Random.Range(5, 10)), Quaternion.identity);
         Destroy(this.gameObject);
     }

     else if (collision.tag == "Enemy" && this.tag == "Laser")
     {
         Instantiate(enemy, new Vector2(Random.Range(-2, 3), Random.Range(5, 10)), Quaternion.identity);
         level.IncreaseScore();
         Destroy(collision.gameObject);
         Destroy(this.gameObject);

     }

     else if (collision.tag == "Enemy" && this.tag == "Player")
     {
         player.playerHealth.Decrement();
     }

     else if (collision.tag == "Shield" && this.tag == "Enemy")
     {
         Instantiate(enemy, new Vector2(Random.Range(-2, 3), Random.Range(5, 10)), Quaternion.identity);
         Destroy(collision.gameObject);
         Destroy(this.gameObject);
     }
 }

}

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

using UnityEngine.UI;

public class Leveling : MonoBehaviour {

 public Text text;
 private int level;
 private int score;
 private int scoreMax;
 private int hp;

 // Start is called before the first frame update

 // Update is called once per frame
 void Start()
 {
     level = 1;
     score = 0;
     scoreMax = 50;
     hp = 3;
 }
 void IncreaseLevel()
 {
     level++;
 }

 public int getHP()
 {
     return hp;
 }

 public void IncreaseScore()
 {
     score = score + 10;

 }

 void Update()
 {
     
     if (score >= scoreMax)
     {
         score = 0;
         scoreMax = scoreMax + 100;
         IncreaseLevel();
     }
     text.text = "Level: " + level;
     
 }

}