Object reference not set when referencing it and adding to array

The Error massage is: NullReferenceException: Object reference not set to an instance of an object GameMaster.OnMapLoaded () (at Assets/_Scripts/GameMaster.cs:19) GameMaster._LoadedMap () (at Assets/_Scripts/GameEvents.cs:15) Grid.Start () (at Assets/_Scripts/Grid.cs:99)

Here are the entire scripts: Grid (should be attached to _GameMaster):

 using System.Collections.Generic;
 using UnityEngine;
 
 public class Grid : MonoBehaviour
 {
     private GameMaster gameMaster;
 
     //The Map
     
     public Tile[,] Map;
 
     //following public variable is used to store the hex model prefab;
     
     public GameObject Hex;
     //next two variables can also be instantiated using unity editor
     public int gridWidthInHexes = 10;
     public int gridHeightInHexes = 10;
 
     //Hexagon tile width and height in game world
     private float hexWidth;
     private float hexHeight;
 
     //Method to initialise Hexagon width and height
     private void setSizes()
     {
         //renderer component attached to the Hex prefab is used to get the current width and height
         hexWidth = Hex.renderer.bounds.size.x;
         hexHeight = Hex.renderer.bounds.size.z;
     }
 
     //Method to calculate the position of the first hexagon tile
     //The center of the hex grid is (0,0,0)
     private Vector3 calcInitPos()
     {
         Vector3 initPos;
         //the initial position will be in the left upper corner
         initPos = new Vector3(-hexWidth*gridWidthInHexes/2f + hexWidth/2, 0,
             gridHeightInHexes/2f*hexHeight - hexHeight/2);
 
         return initPos;
     }
 
     //method used to convert hex grid coordinates to game world coordinates
     public Vector3 calcWorldCoord(Vector2 gridPos)
     {
         //Position of the first hex tile
         Vector3 initPos = calcInitPos();
         //Every second row is offset by half of the tile width
         float offset = 0;
         if (gridPos.y%2 != 0)
             offset = hexWidth/2;
 
         float x = initPos.x + offset + gridPos.x*hexWidth;
         //Every new line is offset in z direction by 3/4 of the hexagon height
         float z = initPos.z - gridPos.y*hexHeight*0.75f;
         return new Vector3(x, 0, z);
     }
 
     //Finally the method which initialises and positions all the tiles
     private void createGrid()
     {
         Map = new Tile[gridWidthInHexes, gridHeightInHexes];
 
         //Game object which is the parent of all the hex tiles
         var hexGridGO = new GameObject("HexGrid");
         int tempTileIndex = 0;
         int curMapIndexHeight = 0;
         for (float y = 0; y < gridHeightInHexes; y++)
         {
             int curMapIndexWidth = 0;
             for (float x = 0; x < gridWidthInHexes; x++)
             {
                 //GameObject assigned to Hex public variable is cloned
                 var hex = (GameObject) Instantiate(Hex);
                 //Current position in grid
                 var gridPos = new Vector2(x, y);
                 var tempTile = hex.GetComponent<Tile>();
                 tempTile.TilePos = gridPos;
                 tempTile.TileIndex = tempTileIndex;
                 hex.transform.position = calcWorldCoord(gridPos);
                 hex.transform.parent = hexGridGO.transform;
                 Map[curMapIndexWidth, curMapIndexHeight] = hex.GetComponent<Tile>();
                 tempTileIndex++;
                 curMapIndexWidth++;
             }
             curMapIndexHeight++;
         }
     }
 
     //The grid should be generated on game start
     private void Start()
     {
         
         gameMaster = gameObject.GetComponent<GameMaster>();
         setSizes();
         createGrid();
         if (gameMaster != null)
         {
             gameMaster._LoadedMap();
         }
     }
 }
 

One of the GameMaster scripts should be attached to _GameMaster :

 using UnityEngine;
 
 public partial class GameMaster : MonoBehaviour
 {
     private Grid grid;
     public GameObject unit;
     void Awake()
     {
         MapLoaded += OnMapLoaded;
     }
     void Start()
     {
         grid = gameObject.GetComponent<Grid>();
     }
 
     void OnMapLoaded()
     {
         Debug.Log("Generated Map");
         Debug.Log(grid.Map[1, 3].TileIndex);
 
         
     }
 
     void Update()
     {
         if (Input.GetMouseButtonDown(0))
         {
             if (MouseClick != null) 
                 MouseClick(0);
         }
 
         if (Input.GetMouseButtonDown(1))
         {
             if (MouseClick != null)
                 MouseClick(1);
         }
 
         if (Input.GetMouseButtonDown(2))
         {
             if (MouseClick != null)
                 MouseClick(2);
         }
     }
 }
 
 public partial class GameMaster
 {
     public delegate void LoadingDeligate();
     public event LoadingDeligate MapLoaded;
 
     public delegate void ClickEvents(int button);
     public event ClickEvents MouseClick;
 
     public void _LoadedMap()
     {
         if (MapLoaded != null) 
             MapLoaded();
     }
 }
 

The tile script is attached to the tile Prefab:

 using UnityEngine;
 
 public class Tile : MonoBehaviour
 {
     private int _tileIndex;
     private Vector2 _tilePos;
     private GameObject _unit;
     private bool isHoverd;
     private GameObject GMObject;
     private GameMaster gameMaster;
     private bool isSelected;
 
     public Texture defultTexture;
     public Texture hoverTexture;
     public Texture selectedTexture;
 
     public int TileIndex
     {
         get { return _tileIndex; }
         set { _tileIndex = value; }
     }
     public Vector2 TilePos
     {
         get { return _tilePos; }
         set
         {
             _tilePos = value; 
             Debug.Log("New TilePos: " + _tilePos);
         }
     }
     public GameObject Unit
     {
         get { return _unit; }
         set { _unit = value; }
     }
 
     void Start ()
     {
         GMObject = GameObject.FindGameObjectsWithTag("GameController")[0];
         gameMaster = GMObject.GetComponent<GameMaster>();
         gameMaster.MouseClick += OnButtonPress;
     }
 
     void OnButtonPress(int button)
     {
         if (isHoverd && button == 0)
         {
             renderer.material.SetTexture("_MainTex", selectedTexture);
             isSelected = true;
         }
         else if (!isHoverd && button == 0 && !Input.GetKey(KeyCode.LeftShift))
         {
             renderer.material.SetTexture("_MainTex", defultTexture);
             isSelected = false;
         }
     }
 
     void OnMouseEnter()
     {
         if (!isSelected)
         {
             renderer.material.SetTexture("_MainTex", hoverTexture);            
         }
         isHoverd = true;
         Debug.Log("hovering: " + _tileIndex);
     }
 
     void OnMouseExit()
     {
         if (!isSelected)
         {
             renderer.material.SetTexture("_MainTex", defultTexture);
         }
         isHoverd = false;
     }
 }
 

Everything works with the exception of accessing the map array and the only error I get is when I'm trying to access the array with the map stored in.

Move your line:

 grid = gameObject.GetComponent<Grid>();

into the Awake method.

OnMapLoaded is most likely being called before the Start method is called, so grid is still null.