Getting a NULL Reference Error inside a constructor C#

Question

Creating a basic snake game.

Trying to get a gameobject to spawn within the limits of a grid, with a sprite. However, when attmpting to call the function that spawns the object, I am getting a NULL REFERENCE error. However, I am confident I have clearly created a reference to the object?

Snake.cs script (Error occurs at line: 'levelGrid.SnakeMoved(gridPosition)')

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

public class Snake : MonoBehaviour
{
    private Vector2Int gridMoveDirection;
    private Vector2Int gridPosition;
    private float gridMoveTimer;
    private float gridMoveTimerMax;
    private LevelGrid levelGrid;

    public void Setup(LevelGrid levelGrid)
    {
        this.levelGrid = levelGrid;
    }


    private void Awake ()
    {
        gridPosition = new Vector2Int(10, 10);
        gridMoveTimerMax = .5f;
        gridMoveTimer = gridMoveTimerMax;
        gridMoveDirection = new Vector2Int(1, 0);
    }

    // Update is called once per frame
    private void Update()
    {
        HandleInput();
        HandleGridMovement();
    }

    private void HandleInput()
    {
        if (Input.GetKeyDown(KeyCode.UpArrow) && gridMoveDirection.y != -1)
        {
            gridMoveDirection.x = 0;
            gridMoveDirection.y = +1;
        }
        if (Input.GetKeyDown(KeyCode.DownArrow) && gridMoveDirection.y != +1)
        {
            gridMoveDirection.x = 0;
            gridMoveDirection.y = -1;
        }
        if (Input.GetKeyDown(KeyCode.LeftArrow) && gridMoveDirection.x != +1)
        {
            gridMoveDirection.x = -1;
            gridMoveDirection.y = 0;
        }
        if (Input.GetKeyDown(KeyCode.RightArrow) && gridMoveDirection.x != -1)
        {
            gridMoveDirection.x = +1;
            gridMoveDirection.y = 0;
        }
    }

    /* Explanation: HandleGridMovement()
     * gridMoveTimer += Time.deltaTime causes the variable of gridMove to incrediment in REAL TIME.
     * gridMoveMaxTime is initalized as the starting value of gridMoveTimer, every 0.5 seconds the if statement will take place
     * the if statement transform both the position of the head of the snake, alongside rotating it accordingly.
     */
    private void HandleGridMovement()
    {

        gridMoveTimer += Time.deltaTime;
        if (gridMoveTimer >= gridMoveTimerMax)
        {
            gridMoveTimer -= gridMoveTimerMax;
            gridPosition += gridMoveDirection;

            transform.position = new Vector3(gridPosition.x, gridPosition.y);
            transform.eulerAngles = new Vector3(0, 0, GetAngleFromVector(gridMoveDirection) - 90);//eurlerAngles r just the rotation angles.

            levelGrid.SnakeMoved(gridPosition);
        }

    }

    private float GetAngleFromVector(Vector2Int dir)
    {
        float n = Mathf.Atan2(dir.y, dir.x) * Mathf.Rad2Deg;
        if (n < 0) n += 360;
        return n;
    }
}

LevelGrid.cs Script: (script that is not working)

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

public class LevelGrid
{

    private Vector2Int foodGridPosition;
    private GameObject foodGameObject;
    private int width;
    private int height;
    private Snake snake;

    public LevelGrid(int width, int height)
    {
        this.width = width;
        this.height = height;

        Debug.Log("here!");
        SpawnFood();
    }

    public void Setup(Snake snake)
    {
        this.snake = snake;
    }

    private void SpawnFood()
    {
        foodGridPosition = new Vector2Int(4,4);

        foodGameObject = new GameObject("FoodApple", typeof(SpriteRenderer));
        foodGameObject.GetComponent<SpriteRenderer>().sprite = GameAssets.i.foodSprite;
        foodGameObject.transform.position = new Vector3(foodGridPosition.x, foodGridPosition.y);
    }

    public void SnakeMoved(Vector2Int snakeGridPosition)
    {
        if (snakeGridPosition == foodGridPosition)
        {
            Object.Destroy(foodGameObject);
            SpawnFood();
        }
    }

}

Other two scripts for clarity sake:

GameHandler.cs

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

public class GameHandler : MonoBehaviour
{

    [SerializeField] private Snake snake;
    
    private LevelGrid levelGrid;


    private void Start()
    {
        Debug.Log("GameHandler Called!");

        levelGrid = new LevelGrid(20, 20);

        snake.Setup(levelGrid);
        levelGrid.Setup(snake);
    }

}

GameAssets.cs

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

public class GameAssets : MonoBehaviour
{

    public static GameAssets i;

    private void Awake()
    {
        i = this;
    }
    
    public Sprite snakeHeadSprite;
    public Sprite foodSprite;
}

Answer 1

The problem is in Update(), but the "cause" is here: snake.Setup(levelGrid).

Unity has no guarantee in what order it will instantiate its objects. Also, Awake() gets called instantly upon creation, followed by Update(), so there is no guarantee that Setup() will be faster.

The best practice approach is doing this in Snake.Update():

private void Update()
{
    if (levelGrid == null)
        return;

    HandleInput();
    HandleGridMovement();
}

When you have variables that depend on some sort of flow logic and are dependent on timings or some external setup, never assume they are assigned. Periodic calls such as frame loops, rendering, physics updates and even repeating invoking has an advantage here as you can "skip" code execution if the code is not ready.

For single-fire functions, that's where you'll need proper design to not get loopholes. Luckily for you, you're in the previous category.

As derHugo said, you should also avoid cross-dependencies from a third dependency, like you have here: snake.Setup(levelGrid); levelGrid.Setup(snake);

While it might work in certain situations, remember that Unity has its own way of instantiating and its own idea of how you should do internal setup (such as Awake(), Start() or OnEnable() depending on the class and situation).