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

I know this is a dumb question to ask and I know that others have asked it before, but none of the answers helped me. What I'm trying to do is check if my Player objects collides with a Collider that is supposed to kill the Enemy object or with a Collider that is supposed to kill the Player object. My problem is, when I try to call in the methods from other scripts to destroy the empty GameObjects that have BoxColliders and the Enemy object, i get this error: NullReferenceException: Object reference not set to an instance of an object.

PlayerMovement.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMovement : MonoBehaviour
 {
     public float SPD_Move = 5F;                                        //Movement speed
     public float JMP_Height = 600F;                                    //Jump height, high due to strong gravitational pull
     public float SPD_Dash;                                             //Dashing speed, needs to be high due to friction or something idc
     private float Dash_Cooldown;                                       //Dashing cooldown
     public int Coins;                                                  //Coins collected
     public int MaxCoins;                                               //Total coins on level
     public int Enemies;                                                //Enemies killed
     public int MaxEnemies;                                             //Total enemies on level
     private bool IsGrounded = true;                                    //Is player on ground?
     public int MaxJumps = 2;                                           //Max jumps, for multiple jumps
     private int CurrentJumps = 0;                                      //Current jumps, resets when in contact with ground
     public float GravitationalPull;                                    //Gravitational force
     private ConstantForce Gravity;                                     //Constant force, applies gravitational pull
     private EnemyMovement EnemyDie;                                    //Supposedly to be called in to destroy enemy
     private PlayerKillerBox PlayerKiller;                              //Called in to destroy the killbox that kills the player
     private EnemyKillerBox EnemyKiller;                                //Called in to destroy the killbox that kills the enemy
 
     private void Start()
     {
         //Decreasing the cooldown every second
         InvokeRepeating("CooldownDecrement", 0F, 1F);
 
         //Disabling built-in gravity
         gameObject.GetComponent<Rigidbody>().useGravity = false;
 
         //Custom gravity
         Gravity = gameObject.AddComponent<ConstantForce>();
         Gravity.force = new Vector3(0, GravitationalPull, 0);
     }
 
     private void Update()
     {
         //Checking if all coins have been collected
         if (Coins == MaxCoins && Enemies == MaxEnemies)
             CurrentLevelData.NextLevel();
 
         //Reset button, if you mess up so badly that you need it xd
         if (Input.GetKeyDown(KeyCode.R))
             CurrentLevelData.ReloadLevel();
     }
 
     private void FixedUpdate()
     {
         //Movement
         if (Input.GetAxis("Horizontal") > 0)
         {
             transform.position += transform.right * Time.deltaTime * SPD_Move;
 
             //Dashing right and reseting the cooldown to 3 seconds
             if ((Input.GetKeyDown(KeyCode.LeftShift) || Input.GetKeyDown(KeyCode.RightShift)) && Dash_Cooldown <= 0)
             {
                 GetComponent<Rigidbody>().AddForce(Vector3.right * SPD_Dash);
                 Dash_Cooldown = 3;
             }
         }
         else if (Input.GetAxis("Horizontal") < 0)
         {
             transform.position -= transform.right * Time.deltaTime * SPD_Move;
 
             //Dashing left and reseting the cooldown to 3 seconds
             if ((Input.GetKeyDown(KeyCode.LeftShift) || Input.GetKeyDown(KeyCode.RightShift)) && Dash_Cooldown <= 0)
             {
                 GetComponent<Rigidbody>().AddForce(Vector3.left * SPD_Dash);
                 Dash_Cooldown = 3;
             }
         }
 
         //Jumping
         if (Input.GetKeyDown(KeyCode.Space) && (IsGrounded || CurrentJumps < MaxJumps))
         {
             //transform.position += transform.up * JMP_Height;    BAD JUMPING, TELEPORTS THROUGH WALLS
             GetComponent<Rigidbody>().AddForce(Vector3.up * JMP_Height);
             IsGrounded = false;
             CurrentJumps++;
         }
 
     }
 
     private void OnCollisionEnter(Collision other)
     {
         //If player is on ground, allow to jump
         if (other.gameObject.tag == "Ground")
         {
             IsGrounded = true;
             CurrentJumps = 0;
         }
 
         //If player collects a coin, increase number of collected coins and destroy the collected coin
         if (other.gameObject.tag == "CoinTag")
         {
             Coins++;
             Destroy(other.gameObject);
         }
 
         //If player jumps on top of enemy, kill the enemy
         if (other.gameObject.tag == "KillEnemy")
         {
             //  Destroy(other.gameObject);
             PlayerKiller.DestroyKillbox();       //Destroys the enemy object
             EnemyKiller.DestroyKillbox();        //Destroys the killbox that kills the enemy
             EnemyDie.DestroyEnemy();             //Destroys enemy object
             Enemies++;
         }
 
         //If player falls out of the map or touches an enemy from the side, reset the level
         if (other.gameObject.tag == "Killbox")
             CurrentLevelData.ReloadLevel();
     }
 
     private void OnTriggerEnter(Collider other)
     {
     }
 
     //Function to be called every second to decrease the cooldowns
     private void CooldownDecrement()
     {
         if (Dash_Cooldown > 0)
             Dash_Cooldown--;
     }
 }
 

EnemyMovement.cs

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class EnemyMovement : MonoBehaviour
 {
     
 
     public void DestroyEnemy()
     {
         Destroy(this.gameObject);
     }
 }

PlayerKillerBox.cs, which has the method for destroying the GameObject that holds the Collider which kills the player

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerKillerBox : MonoBehaviour {
 
     public void DestroyKillbox()
     {
         Destroy(gameObject);
     }
 }

EnemyKillerBox.cs which has the method for destroying the GameObject holding the Collider which destroys the enemy object

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class EnemyKillerBox : MonoBehaviour {
 
     public void DestroyKillbox()
     {
         Destroy(gameObject);
     }
 }

Any help is greatly appreciated and once again sorry for posting what has been posted before. Thank you very much in advance!