Problem with NullReferenceException in the course

Unity Courses Ask
,
#1

Hi all. I’ve been following Hugo for a while now and this lecture was a logical step. I have followed everything “by the book” but when I start to test the game in this lecture I get

NullReferenceException: Object reference not set to an instance of an object
Unit.Start () (at Assets/Scripts/Unit.cs:18)

for this line

gridPosition = LevelGrid.Instance.GetGridPosition(transform.position);

and this error

NullReferenceException: Object reference not set to an instance of an object
Unit.Update () (at Assets/Scripts/Unit.cs:39)

for this line

GridPosition newGridPosition = LevelGrid.Instance.GetGridPosition(transform.position);
if (newGridPosition != gridPosition) {
// Unit changed Grid Position
LevelGrid.Instance.UnitMovedGridPosition(this, gridPosition, newGridPosition);
gridPosition = newGridPosition;
}

any idea what am I missing?

#2

Found the issue but now have another. The issue was in LevelGrid.cs I did not actually set the instance on awake. but now it only works for bottom row. “Height” grid spaces are not working. Any idea?

#3

I’m not entirely sure what your meaning with the “Height”… I’m assuming you’re not meaning the floor (from the bonus lectures, multi-floor play).

Are you saying that it’s only retrieving gridPositions with a gridPosition.z value of 0 (the first “row”)?

Let’s take a look at your LevelGrid.cs and your GridSystem.cs scripts.

#4

Yes thats prob it. Here are the codes.

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

public class LevelGrid : MonoBehaviour {

    public static LevelGrid Instance { get; private set; }

    [SerializeField] private Transform gridDebugObjectPrefab;
    private GridSystem gridSystem;

    private void Awake() {

        if (Instance != null) {
            Debug.LogError("There is more than one LevelGrid!" + transform + " - " + Instance);
            Destroy(gameObject);
            return;
        }

        Instance = this;

        gridSystem = new GridSystem(10, 10, 2f);
        gridSystem.CreateDebugObjects(gridDebugObjectPrefab);
    }


    public void AddUnitAtGridPosition(GridPosition gridPosition, Unit unit) {
        GridObject gridObject = gridSystem.GetGridObject(gridPosition);
        gridObject.AddUnit(unit);
    }

    public List<Unit> GetUnitListAtGridPosition(GridPosition gridPosition) {
        GridObject gridObject = gridSystem.GetGridObject(gridPosition);
        return gridObject.GetUnitList();
    }

    public void RemoveUnitAtGridPosition(GridPosition gridPosition, Unit unit) {
        GridObject gridObject = gridSystem.GetGridObject(gridPosition);
        gridObject.RemoveUnit(unit);
    }

    public void UnitMovedGridPosition(Unit unit, GridPosition fromGridPosition, GridPosition toGridPosiiton) {
        RemoveUnitAtGridPosition(fromGridPosition, unit);

        AddUnitAtGridPosition(toGridPosiiton, unit);
    }

    public GridPosition GetGridPosition(Vector3 worldPosition) => gridSystem.GetGridPosition(worldPosition);
}

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

public class GridSystem {


    private int width;
    private int height;
    private float cellSize;
    private GridObject[,] gridObjectArray;

    public GridSystem(int width, int height, float cellSize) {
        this.width = width;
        this.height = height;
        this.cellSize = cellSize;

        gridObjectArray = new GridObject[width, height];
        for (int x = 0; x < width; x++) { 
            for (int z = 0; z < height; z++) {
                GridPosition gridPosition = new GridPosition(x, z);
                gridObjectArray[x, z] = new GridObject(this, gridPosition);
            }
        }
        
    }

    public Vector3 GetWorldPosition(GridPosition gridPosition) {
        return new Vector3(gridPosition.x, 0, gridPosition.z) * cellSize;
    }
    
    public GridPosition GetGridPosition(Vector3 worldPosition) {
        return new GridPosition(
                Mathf.RoundToInt(worldPosition.x / cellSize),
                Mathf.RoundToInt(worldPosition.y / cellSize)
            );
    }
    
    public void CreateDebugObjects(Transform debugPrefab) {
        for (int x = 0; x < width; x++) {
            for (int z = 0; z < height; z++) {
                GridPosition gridPosition = new GridPosition(x, z);

                Transform debugTransform = GameObject.Instantiate(debugPrefab, GetWorldPosition(gridPosition), Quaternion.identity);
                GridDebugObject gridDebugObject = debugTransform.GetComponent<GridDebugObject>();
                gridDebugObject.SetGridObject(GetGridObject(gridPosition));
            }
        }
    }

    public GridObject GetGridObject(GridPosition gridPosition) {
        return gridObjectArray[gridPosition.x, gridPosition.z];
    }

}
#5

This looks correct. Are the GridDebugObjects populating along the full grid?

One more check, paste in your GridPosition.cs

For code pastes, you use the single ` for inline code, for example `code` will become code
For code blocks, use three ``` on a line by themselves at the beginning and end of the code block
so
```
This is some code
In a block
```
becomes

This is some code
In a block

I edited your prior posts to reflect this. If you edit the post, you can see what I’ve done.

#6

Thanks.

Course
Course1070×436 559 KB

Maybe you can see it here more clearly. it registers second unit in first row. other rows dont work.
Here is code:




using System;

public struct GridPosition : IEquatable<GridPosition> {
    public int x;
    public int z;

    public GridPosition(int x, int z) {

        this.x = x;
        this.z = z;
    }

    public override bool Equals(object obj) {
        return obj is GridPosition position &&
               x == position.x &&
               z == position.z;
    }

    public bool Equals(GridPosition other) {
        return this == other;
    }

    public override int GetHashCode() {
        return HashCode.Combine(x, z);
    }

    public override string ToString() {
        return $"x: {x}; z: {z}";
    }

    public static bool operator ==(GridPosition a, GridPosition b) {
        return a.x == b.x && a.z == b.z ;
    }

    public static bool operator !=(GridPosition a, GridPosition b) 
        { return !(a == b); }

}
#7

The problem is here. See if you can spot it. You’re using worldPosition.y instead of worldPosition.z (sorry for poaching your post, @Brian_Trotter)

#8

Yes I can… Thank you very much. Looks like I auto-complete it and did not double check.

prob was in worldPosition.y
correct func would be
Mathf.RoundToInt(worldPosition.z / cellSize)

Thank you very much, got it running again!

#9

On a unrelated note - is it possible to get private coaching from someone here at gamedev.tv? I have a problem on a different project I was working and I literally quit the project and went back to learning because of it. It would be nice to have an option for 1 on 1 coaching, for a fee of course, just to be able to pass some roadblock easier. I have 2 small kids and 2 jobs so my time is limited and I can’t really afford going through ton of Unity docs when I don’t really know what it is I am searching for :smiley:

#10

Finally spotted it. GetGridPosition() in GridSystem, you’re using worldPosition.y instead of worldPosition.z
Apologies for not catching that on the first go. Once I saw the picture, it spellled out what the problem must be.

#11

LOL, somehow my answer came a bit late. Didn’t noticed that @bixarrio already had it well in hand.

#12

That’s ok. Thank you very much for quick response to my problem. I know it looks silly but I am still quite new to it all and simple mistakes like this still do happen from time to time.

#13

Yep, and that one is an easy mistake to make. Don’t let the fancy Teaching Assistant title fool ya. Us instructors make mistakes too. :slight_smile:

#14
#15

This topic was automatically closed 24 hours after the last reply. New replies are no longer allowed.

Privacy & Terms