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

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[i];
 
             // 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<ObjectPooler>().prefab = prefabType;
             newPool.GetComponent<ObjectPooler>().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<ObjectPooler>().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<GameObject> pooledObjects;
 
     void Start()
     {
         // pooledObjects is a list of Game Objects to be pooled.
         pooledObjects = new List<GameObject>();
 
         // 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[i].activeInHierarchy)
             {
                 return pooledObjects[i];
             }
         }
 
         // 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...