How to fix this error in Unity3d?

Question

Unity

I am learning in Unity3d from a book. I am writing the code, but I still get this error:

NullReferenceException: Object reference not set to an instance of an object
GateKeeper.OnCollisionStay (UnityEngine.Collision coll) (at Assets/__Scripts/GateKeeper.cs:28)


The code was rewritten correctly. It should work like this:
the GG comes to the door and she should change the Tile to an open door, but instead, when colliding with ANY Tile, it gives the above error.

Here is the code where the collision is involved:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class GateKeeper : MonoBehaviour
{
    const int lockedR = 95;
    const int lockedUR = 81;
    const int lockedUL = 80;
    const int lockedL = 100;
    const int lockedDL =101;
    const int lockedDR = 102;

    const int openR = 48;
    const int openUR = 93;
    const int openUL = 92;
    const int openL = 51;
    const int openDL  =26;
    const int openDR = 27;

    private IKeyMaster keys;

    void Awake(){
        keys = GetComponent<IKeyMaster>();
    }

    void OnCollisionStay( Collision coll ){
        if (keys.keyCount < 1) return;

        Tile ti = coll.gameObject.GetComponent<Tile>();
        if (ti == null) return;

        int facing = keys.GetFacing();
        Tile ti2;
        switch (ti.tileNum) {
            case lockedR:
                if(facing != 0) return;
                ti.SetTile(ti.x , ti.y , openR);
                break;
            case lockedUR:
                if(facing != 1) return;
                ti.SetTile( ti.x, ti.y, openUR );
                ti2 = TileCamera.TILES[ti.x-1, ti.y];
                ti2.SetTile( ti2.x, ti2.y, openUL );
                break;
            case lockedUL:
                if (facing != 1) return;
                ti.SetTile( ti.x, ti.y, openUL );
                ti2 = TileCamera.TILES[ti.x+1, ti.y];
                ti2.SetTile( ti2.x, ti2.y, openUR );
                break;
            case lockedL:
                if (facing != 2) return;
                ti.SetTile( ti.x, ti.y, openL );
                break;
            case lockedDL:
                if (facing != 3) return;
                ti.SetTile( ti.x, ti.y, openDL );
                ti2 = TileCamera.TILES[ti.x+1, ti.y];
                ti2.SetTile( ti2.x, ti2.y, openDR );
                break;
            case lockedDR:
                if (facing != 3) return;
                ti.SetTile( ti.x, ti.y, openDR );
                ti2 = TileCamera.TILES[ti.x-1, ti.y];
                ti2.SetTile( ti2.x, ti2.y, openDL );
                break;
            
            default:
                return;
        }
        keys.keyCount--;
    }
}

This is the Tile code (map generation, etc.)

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Tile : MonoBehaviour
{
    [Header("Set Dinamically")]
    public int x;
    public int y;
    public int tileNum;

    private BoxCollider bColl;

    void Awake(){
        bColl = GetComponent<BoxCollider>();
    }

    public void SetTile(int eX , int eY , int eTileNum = -1){
        x = eX;
        y = eY;
        transform.localPosition = new Vector3(x , y , 0);
        gameObject.name = x.ToString("D3") + "x" + y.ToString("D3");

        if(eTileNum == -1){
            eTileNum = TileCamera.GET_MAP(x , y);
        }else {
            TileCamera.SET_MAP(x ,  y, eTileNum); 
        }
        tileNum = eTileNum;
        GetComponent<SpriteRenderer>().sprite = TileCamera.SPRITES[tileNum];

        SetCollider();
    }

    void SetCollider(){
        bColl.enabled = true;
        char c = TileCamera.COLLISIONS[tileNum];
        switch (c){
            case 'S':
                bColl.center = Vector3.zero;
                bColl.size = Vector3.one;
                break;
            case 'W':
                bColl.center = new Vector3( 0, 0.25f, 0 );
                bColl.size = new Vector3( 1, 0.5f, 1 );
                break;
            case 'A':
                bColl.center = new Vector3( -0.25f, 0, 0 );
                bColl.size = new Vector3( 0.5f, 1, 1 );
                break;
            case 'D':
                bColl.center = new Vector3(0.25f, 0 ,0);
                bColl.size = new Vector3( 0.5f, 1, 1 );
                break;
            case 'Q':
                bColl.center = new Vector3( -0.25f, 0.25f, 0 );
                bColl.size = new Vector3( 0.5f, 0.5f, 1 );
                break;
            case 'E':
                bColl.center = new Vector3( 0.25f, 0.25f, 0 );
                bColl.size = new Vector3( 0.5f, 0.5f, 1 );
                break;
            case 'Z':
                bColl.center = new Vector3( -0.25f, -0.25f, 0 );
                bColl.size = new Vector3( 0.5f, 0.5f, 1 );
                break;
            case 'X':
                bColl.center = new Vector3( 0, -0.25f, 0 );
                bColl.size = new Vector3( 1, 0.5f, 1 );
                break;
            case 'C':
                bColl.center = new Vector3( 0.25f, -0.25f, 0 );
                bColl.size = new Vector3( 0.5f, 0.5f, 1 );
                break;
            default:
                bColl.enabled = false;
                break;
        }
    }
}

This is the interface code for Door Keys.:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public interface IKeyMaster
{
    int keyCount { get; set; }
    int GetFacing();
}

I ask for help, please.