Accessing object that don't exist

Unity Engine
,
1

I want enemy moving from point a to point b, and after reaching point b this enemy will be deleted. Works almost fine, enemies are moving ect. but i see this:

MissingReferenceException: The object of type ‘Transform’ has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.Object.Internal_InstantiateSingle (UnityEngine.Object data, UnityEngine.Vector3 pos, UnityEngine.Quaternion rot) (at :0)
UnityEngine.Object.Instantiate (UnityEngine.Object original, UnityEngine.Vector3 position, UnityEngine.Quaternion rotation) (at :0)
UnityEngine.Object.Instantiate[T] (T original, UnityEngine.Vector3 position, UnityEngine.Quaternion rotation) (at :0)
WaveSpawner.SpawnEnemy () (at Assets/Scripts/WaveSpawner.cs:39)
WaveSpawner+d__6.MoveNext () (at Assets/Scripts/WaveSpawner.cs:32)
UnityEngine.SetupCoroutine.InvokeMoveNext (System.Collections.IEnumerator enumerator, System.IntPtr returnValueAddress) (at :0)
UnityEngine.MonoBehaviour:StartCoroutine(IEnumerator)
WaveSpawner:Update() (at Assets/Scripts/WaveSpawner.cs:19)

After that enemies stops spawning. Enemies spawned earlier moves correctly, but after reaching its destination same error appear.

Enemy code:

using System.Diagnostics;
using UnityEngine;

public class BasicEnemy : MonoBehaviour {

    public float speed = 10f;

    private Transform target;
    private int wavepointIndex = 0;

    void Start()
    {
        target = Waypoints.points[0];
    }

    void Update()
    {
        Vector3 dir = target.position - transform.position;
        transform.Translate(dir.normalized * speed * Time.deltaTime, Space.World);

        if (Vector3.Distance(transform.position, target.position) <= 0.4f)
        {
            GetNextWaypoint();
        }

    }
    void GetNextWaypoint()
    {
        if (wavepointIndex >= Waypoints.points.Length - 1)
        {
            Destroy(gameObject);
            return;
           
        }
        wavepointIndex++;
        target = Waypoints.points[wavepointIndex];
    }

}

And code for spawning enemies:

using UnityEngine;
using System.Collections;

public class WaveSpawner : MonoBehaviour {

    public Transform enemyPrefab;

    public Transform spawnPoint;

    public float timeBetweenWaves = 5f;
    private float countdown = 2f;

    private int waveIndex = 0;

    void Update ()
    {
        if (countdown <= 0f)
        {
            StartCoroutine(SpawnWave());
            countdown = timeBetweenWaves;
        }

        countdown -= Time.deltaTime;
    }

    IEnumerator SpawnWave ()
    {
        waveIndex++;

        for (int i = 0; i < waveIndex; i++)
        {
            SpawnEnemy();
            yield return new WaitForSeconds(0.5f);
        }
    }
   
    void SpawnEnemy ()
    {
        Instantiate(enemyPrefab, spawnPoint.position, spawnPoint.rotation);
    }
}

I’m new at unity, so please explain why it should be done other way. Thanks, and i hope your eyes still exist after my english…

2

For your WaveSpawner object:

Presumably you have dragged some value into spawnEnemy field in the editor, correct?

This object you dragged in, is it a real enemy object in the scene? Or is it a prefab from the project window? If it’s a real enemy from the scene, your problem is that when that enemy destroys itself when it reaches the waypoint, the WaveSpawner can no longer copy it because it is destroyed. You should use a prefab instead.

3

Some notes on how to fix a NullReferenceException in Unity3D (also Unassigned Reference errors):

http://plbm.com/?p=221

The basic steps outlined above are:

  • Identify what is null
  • Identify why it is null
  • Fix that.
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