Null Reference Exception ONLY (sometimes) when loading the scene...

Questions & Answers
1

Trying to set up a simple menu, which works. I load my “main scene” with the following script attached to a button:

public class ChangeScene : MonoBehaviour {

	public void ChangeToScene (string sceneToChangeTo)
    {
        SceneManager.LoadScene(sceneToChangeTo);
    }
}

This all works. The issue is, when I use that to load my “Main Scene”, I get a Null Reference Exception error.

NullReferenceException: Object reference not set to an instance of an object
ObjectPooler.GetPooledObject () (at Assets/Scripts/ObjectPooler.cs:33)
WorldGenerator.Update () (at Assets/Scripts/WorldGenerator.cs:51)

This only occurs sometimes. If I play the scene directly from within that scene, it never occurs, but if I load it by clicking on a button it will occur around 70% of the time. As far as I can tell, it doesn’t actually affect anything in the game, and only triggers once at the start, but I’d still like to fix it.

Here is the world generator script:

using System.Collections.Generic;
using UnityEngine;

public class WorldGenerator : MonoBehaviour
{
    public int worldType;

	public Transform generationPoint;
	public float distanceBetween;

    public CreateLevelSO[] levelsSO;
    public GameObject pooler;
    public GameObject poolFolder;
    public GameObject obstacleFolder;

    List<GameObject> objectPools;


    void Start ()
    {
        // List of pools.
        objectPools = new List<GameObject>();

        // Iterate through all prefabs on SO.
        for (int i = 0; i < levelsSO[worldType].Prefabs.Length; i++)
        {
            // Get the game object held on the SO.
            GameObject prefabType = levelsSO[worldType].Prefabs*;*

// Instantiate a pool under the correct folder.
GameObject newPool = Instantiate(pooler, poolFolder.transform);

// Set the properties of the new object and add it to the list.
newPool.GetComponent().prefab = prefabType;
newPool.GetComponent().obstacleFolder = obstacleFolder;
objectPools.Add(newPool);
}
}

void Update ()

  • {*
  •  if (transform.position.z < generationPoint.position.z)* 

  •  {*

  •  	transform.position = new Vector3(transform.position.x, transform.position.y, transform.position.z + distanceBetween);*

// Randomly select obstacle.
int type = Random.Range(0, levelsSO[worldType].Prefabs.Length);

// Get pooled object from the correct object pool.
GameObject newObstacle = objectPools[type].GetComponent().GetPooledObject();

// Activate and move recieved object.
newObstacle.transform.position = transform.position;
newObstacle.SetActive(true);

  •  }	*
  • }*
    }
    Here is the object pooler script:
    using System.Collections.Generic;
    using UnityEngine;

public class ObjectPooler : MonoBehaviour
{

public GameObject prefab;
public GameObject obstacleFolder;
public int initialPooledAmount;

List pooledObjects;

void Start()
{
// pooledObjects is a list of Game Objects to be pooled.
pooledObjects = new List();

// Instantiate the initial pool.
for (int i = 0; i < initialPooledAmount; i++)
{
GameObject newObject = Instantiate(prefab, obstacleFolder.transform);
newObject.SetActive(false);
pooledObjects.Add(newObject);
}
}

// Function called to return an available pooled object.
public GameObject GetPooledObject()
{

// Iterates through current pooled objects, returning the first deactivated object.
for(int i = 0; i < pooledObjects.Count; i++)
{
if(!pooledObjects*.activeInHierarchy)*
{
return pooledObjects*;*
}
}

// If there are no deactivated objects, expand the pool and return it.
GameObject newObject = Instantiate(prefab, obstacleFolder.transform);
newObject.SetActive(false);
pooledObjects.Add(newObject);
return newObject;

}
}
Any help is appreciated, this is incredibly confusing for me…

2

My best guess, looking at the code, is that your WorldGenerator.Update()'s first call happens before all of the ObjectPooler.Start() functions have been called.

Honestly, I would remove the Start() function from ObjectPooler entirely and add this to the beginning of the GetPooledObject() function:

if(pooledObjects == null)
    pooledObjects = new List<GameObject>();