Resolved "Object reference not set to an instance of an object" error

Hello there! I have a problem where I get this error :
NullReferenceException: Object reference not set to an instance of an object
SpawnRoom.OnTriggerEnter2D (UnityEngine.Collider2D collision) (at Assets/Scripts/SpawnRoom.cs:12)

I have 2 scripts one named "SpawnRoom" and "NormalRoomLists".
"SpawnRoom" would take the list from "NormalRoomList" and Instantiate it.

"SpawnRoom"

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4. public class SpawnRoom : MonoBehaviour
5. {
6.     public NormalRoomLists normalRoomLists;
7.     private List<GameObject> normalRooms;
8.     private void OnTriggerEnter2D(Collider2D collision)
9.     {
10.         if (collision.CompareTag("Player"))
11.         {
12.             normalRooms = normalRoomLists.normalRooms;
13.             int prefabIndex = UnityEngine.Random.Range(0, normalRooms.Count -1);
14.             Instantiate(normalRooms[prefabIndex], transform.position, transform.rotation);
15.         }
16.     }
17. }

"NormalRoomList"

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. [System.Serializable]
6. public class NormalRoomLists : MonoBehaviour
7. {
8.     public GameObject Prefab1;
9.     public GameObject Prefab2;
10.     public GameObject Prefab3;
11.  
12.     public List<GameObject> normalRooms = new List<GameObject>();
13.  
14.     void Start()
15.     {
16.         normalRooms.Add(Prefab1);
17.         normalRooms.Add(Prefab2);
18.         normalRooms.Add(Prefab3);
19.     }
20. }

When I touch the trigger I get the error shown above.

 

The answer is always the same... ALWAYS. It is the single most common error ever.

Don't waste your life spinning around and round on this error. Instead, learn how to fix it fast... it's EASY!!

Some notes on how to fix a NullReferenceException error in Unity3D
- also known as: Unassigned Reference Exception
- also known as: Missing Reference Exception
- also known as: Object reference not set to an instance of an object

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

The basic steps outlined above are:
- Identify what is null
- Identify why it is null
- Fix that.

Expect to see this error a LOT. It's easily the most common thing to do when working. Learn how to fix it rapidly. It's easy. See the above link for more tips.

You need to figure out HOW that variable is supposed to get its initial value. There are many ways in Unity. In order of likelihood, it might be ONE of the following:

- drag it in using the inspector
- code inside this script initializes it
- some OTHER external code initializes it
- ? something else?

This is the kind of mindset and thinking process you need to bring to this problem:

https://forum.unity.com/threads/why-do-my-music-ignore-the-sliders.993849/#post-6453695

Step by step, break it down, find the problem.

Here is a clean analogy of the actual underlying problem of a null reference exception:

https://forum.unity.com/threads/nul...n-instance-of-an-object.1108865/#post-7137032

 

Ah I got it fixed, and I feel so dumb. I forgot to assign the NormalRoomLists script to the SpawnRoom. Thanks for helping!