Get XP on enemy death

Hello.

I was stucked with some weird problem for 2 days. Sometime ago I started to implement level system. I wrote it and wanted to write script for gaining experience on enemy death. I used event system to do this and subscribe to it, but I simply can't trigger it in game upon enemy death.

Unity says: "Object reference not set to an instance of an object" at line 24 in BloodSucker script.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System;
 
 public class BloodSucker : MonoBehaviour {
 
     public int health;
     public GameObject effect;
     public GameObject corpse;
 
     public event EventHandler OnBloodSuckerKilled;
 
     public LevelSystem levelSystem;
 
     private void Update()
     {
         if (health <= 0) 
         {
             Instantiate(corpse, transform.position, Quaternion.identity);
             Instantiate(effect, transform.position, Quaternion.identity);
             Destroy(gameObject);
 
             OnBloodSuckerKilled.Invoke(levelSystem, EventArgs.Empty);
         }
     }
 
     public void TakeDamage(int damage) {
         health -= damage;
     }
 
     public void XP_OnBloodSuckerKilled(object sender, System.EventArgs e) {
         levelSystem.AddExperience(100);
         Debug.Log("This code working");
     }
 
     public void SetLevelSystem(LevelSystem levelSystem)
     {
         this.levelSystem = levelSystem;
 
         OnBloodSuckerKilled += XP_OnBloodSuckerKilled;
     }
 }

You can say that I should make new LevelSystem, but I can't, cause I have one more script that using my levelSystem, so I've created code that sends them original levelSystem in SetLevelSystem

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class TestingLvlSys : MonoBehaviour
 {
      public ExperienceBarUI experienceBarUI;
      public BloodSucker bloodSucker;
 
      LevelSystem levelSystem = new LevelSystem();
 
     public void Awake()
     {
         experienceBarUI.SetLevelSystem(levelSystem);
         bloodSucker.SetLevelSystem(levelSystem);
     }
 }

So, I need to Invoke my XP_OnBloodSuckerKilled when enemy killed. How can I do that. Thank you in advance for answers.