Level Generator Issue

I have a really weird issue in my LevelGenerator script. Pretty much I’ve been following a level generating tutorial by CodeMonkey and everything was working completely fine and the script was working perfectly. I then wanted to move the exact same script into a different project that I’ve been working on but then I’ve been hit with the following error. (didn’t change anything at all about the script in the other project)

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ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
System.ThrowHelper.ThrowArgumentOutOfRangeException (System.ExceptionArgument argument, System.ExceptionResource resource) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.ThrowHelper.ThrowArgumentOutOfRangeException () (at <437ba245d8404784b9fbab9b439ac908>:0)
LevelGenerator.SpawnLevelPart () (at Assets/Scripts/LevelGenerator.cs:72)
LevelGenerator.Awake () (at Assets/Scripts/LevelGenerator.cs:39)

The strange thing is is that it works perfectly fine in one project but spits out this error in a different project. Any help on this issue would be appreciated.

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

public class LevelGenerator : MonoBehaviour
{
    private const float PLAYER_DISTANCE_SPAWN_LEVEL_PART = 10f;

    [SerializeField] private Transform levelPart_Start;
    [SerializeField] private Transform pfTestingPlatform;
    [SerializeField] private List<Transform> levelPartEasyList;
    [SerializeField] private List<Transform> levelPartMediumList;
    [SerializeField] private List<Transform> levelPartHardList;
    [SerializeField] private List<Transform> levelPartExtremeList;
    [SerializeField] private PlayerController player;

    private enum Difficulty
    {
        Easy,
        Medium,
        Hard,
        Extreme
    }
    private Vector3 lastEndPosition;
    private int levelsPartsSpawned;

    private void Awake()
    {
        lastEndPosition = levelPart_Start.Find("EndPosition").position;

        if(pfTestingPlatform !=null)
        {
            Debug.Log("Debugging Platforms");
        }
       
        int startingSpawnLevelParts = 5;
        for(int i = 0; i < startingSpawnLevelParts; i++)
        {
            SpawnLevelPart();
        }
    }

    private void Update()
    {
        if(Vector3.Distance(player.GetPosition(), lastEndPosition) < PLAYER_DISTANCE_SPAWN_LEVEL_PART)
        {
            SpawnLevelPart();
        }
    }

    private void SpawnLevelPart()
    {
        List <Transform> difficultyLevelPartList;
        switch(GetDifficulty())
        {
            default:
            case Difficulty.Easy: difficultyLevelPartList = levelPartEasyList; break;
            case Difficulty.Medium: difficultyLevelPartList = levelPartMediumList; break;
            case Difficulty.Hard: difficultyLevelPartList = levelPartHardList; break;
            case Difficulty.Extreme: difficultyLevelPartList = levelPartExtremeList; break;
        }         
      
        Transform chosenLevelPart = difficultyLevelPartList[Random.Range(0, difficultyLevelPartList.Count)];

        if (pfTestingPlatform != null)
        {
            chosenLevelPart = pfTestingPlatform;
        }
        Transform lastLevelPartTransform = SpawnLevelPart(chosenLevelPart, lastEndPosition);
        lastEndPosition = lastLevelPartTransform.Find("EndPosition").position;
        levelsPartsSpawned++;
    }
   
    private Transform SpawnLevelPart(Transform levelPart, Vector3 spawnPosition)
    {
        Transform levelPartTransform = Instantiate(levelPart, spawnPosition, Quaternion.identity);
        return levelPartTransform;
    }

    private Difficulty GetDifficulty()
    {
        if (levelsPartsSpawned >= 15) return Difficulty.Extreme;
        if (levelsPartsSpawned >= 10) return Difficulty.Hard;
        if (levelsPartsSpawned >= 5) return Difficulty.Medium;
        return Difficulty.Easy;
    }
}

I’ve restarted my computer and suddenly the issue fixed itself.