Error updating UI text when instantiated object is destroyed

Unity Engine
1

Hello all,

I’m new to Unity and scripting in C# and I have encountered a problem.

Desierd behaviour.
At the start of the game, asteroids are instantiated by s_GameManager.cs.
When the player shoots an asteroid the s_asteroid.cs sends an int-value (points) to the s_ScoreDisplay.cs.
s_ScoreDisplay.cs should update the Score.text, currently a child gameobject to the UI (Canvas).

Error
When an asteroid is destroyed, an error is encountered at line 19, where the s_Asteroid.cs sends points to the s_Scoredisplay.cs.

Unity Console error:
NullReferenceException: Object reference not set to an instance of an object
s_Asteroid.OnTriggerEnter2D (UnityEngine.Collider2D collider) (at Assets/Scripts/s_Asteroid.cs:19)

Scripts:
s_GameManager.cs

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

public class s_GameManager : MonoBehaviour {

    public GameObject Asteroid;

    // Use this for initialization
    private void Start()
    {
        for (int i = 0; i <= 10; i++) //number of asteroids to be instansiated at beggining of level
        {
            Vector3 randomPosistion = new Vector3(Random.Range(-20f, 20f), Random.Range(-10f, 10f), -1f); //Random position for asteroid to spawn on
            Instantiate(Asteroid, randomPosistion, Quaternion.identity); //Create asteroid at random position
        }
    }

    // Update is called once per frame
    void Update () {
      
    }
}

s_Asteroid.cs

using UnityEngine;

public class s_Asteroid : MonoBehaviour
{
    private float Health = 15f;
    private s_ScoreDisplay scoreDisplay;
    private int points = 10;

    public void OnTriggerEnter2D(Collider2D collider)
    {
        if (collider.gameObject.tag == "Projectile") //IF the collision is triggerd by plasma shot
        {
            Destroy(collider.gameObject); //Destroy plasma shot
            Health--;
            if (Health <= 0f) //If asteroids health is below or equal to 0
            {
                Destroy(gameObject); //Destroys asteroid
                Debug.Log("Sending points...");
                scoreDisplay.ScoreCounter(points); //Send points to ScoreDisplay
                Debug.Log("Points sent");
            }
        }
    }
}

s_ScoreDisplay.cs

using UnityEngine;
using UnityEngine.UI;

public class s_ScoreDisplay : MonoBehaviour
{
    private Text scoreText; //Score text
    private int score;

    // Use this for initialization
    public void Start()
    {

        scoreText = GetComponent<Text>();
        score = 0;
        scoreText.text = score.ToString();
    }

    public void ScoreCounter(int points)
    {
        Debug.Log("Entering ScoreCounter()");
        score += points; //Increase score by the recived points from other functions
        Debug.Log("Before error ");
        scoreText.text = score.ToString();
        Debug.Log("End of scoreCounter");
    }
}

I’ve left my debug.log messages and comments so you easier get the idea of my problem.
I tried the forums for a solution and my best bet would be that there is some problem with that the instantiated asteroid can not “find” (reference?) the s_ScoreDisplay.cs.
But I’m at a loss.

Appreciate any help with the error!

Warm regards
Jonathan

2

First you could just send the score and then destroy the asteroid, but your problem is different for the error.

You never assign a value to

private s_ScoreDisplay scoreDisplay;

scoreDisplay has no value, so you get a null error. Either make scoreDisplay a singleton, or create the reference in a gamemanager type script which you could also access through the gamemanager (which would be a singleton)

3

Hi, thank you for your answer.

The reason I declare the private is so that I can call the method in s_ScoreDisplay (ScoreCounter())… Still I will look into your comment and see if I misunderstood it!

Thank you!

Jonathan

4

I don’t see anything in the code that actually assigns scoreDisplay to anything. Its default value is null. So you’ll get a null reference exception when trying to access it.

It has nothing to do with whether it’s private or public.

5

As @BlackPete added. It doesn’t care if it’s private, but you still have to assign a value to the variable. I just tried to give you an option since you may be instantiating your asteroids. If you are using object pooling on your asteroids, you can do something a bit different.

6

Thank you. Now I understand what you both mean. I Debug.Log(scoreDisplay), and I do get a null value. I will look in to it and get will ask more questions if I get lost! Thanks