Unity 2D: NullReferenceException. How to add two manager scripts? - animation

Question

I am quite new to Unity, so please bear with my horrible explanation. I have followed a tutorial on youtube about a 2D fighting game: https://www.youtube.com/watch?v=n8S3WgVoOmo&t=3319s
I will provide my code below.
In the linked video, the video-maker makes one PlayerManager script which controls both my player and duplicates of my player. Through a simple AI, the duplicate becomes my enemy and we fight. This much works as expected.
Now I wanted to change it a bit like a 2D platformer. I took the PlayerManager and split it into two. AIManager (same code as in tutorial overall) and a PlayerManager with some changes. This also worked well and I am able to move and the AI also recognises me. The problem comes from a script called DamageScript. It recognizes if and when I take damage, and triggers the relevant animation.
When the DamageScript was connected to just the PlayerManager, when I hit the AI or AI hit me, whoever got hit showed the Damage animation. But after the split I did, with the separate Managers for AI and Player, I have two options. Either I hit the AI, he does Damage animation and when he hits me I get and error. Or he hits me, I have the Damage animations and when I hit him I get an error. The error is this:
NullReferenceException: Object reference not set to an instance of an object
DamageScript.OnTriggerEnter2D (UnityEngine.Collider2D col) (at Assets/Scripts/DamageScript.cs:19)
This is the original DamageScript:
public class DamageScript : MonoBehaviour {
void OnTriggerEnter2D(Collider2D col)
{
if(col.transform.root != transform.root && col.tag != "Ground" && !col.isTrigger)
{
if (!col.transform.GetComponent<AIManager>().damage && !col.transform.GetComponent<PlayerManager>().blocking)
{
col.transform.GetComponent<AIManager>().damage = true;
col.transform.root.GetComponentInChildren<Animator>().SetTrigger("Damage");
}
}
}
}
This way my Player does the Damage animation, I get the error when I hit. So, I figured something like this would work, but I guess I don't really know how to code it:
void OnTriggerEnter2D(Collider2D col)
{
if(col.transform.root != transform.root && col.tag != "Ground" && !col.isTrigger)
{
line 11 if (!col.transform.GetComponent<AIManager>().damage && !col.transform.GetComponent<AIManager>().blocking)
{
col.transform.GetComponent<AIManager>().damage = true;
col.transform.root.GetComponentInChildren<Animator>().SetTrigger("Damage");
}
else
{
if (!col.transform.GetComponent<PlayerManager>().damage)
{
line 19 col.transform.GetComponent<PlayerManager>().damage = true;
col.transform.root.GetComponentInChildren<Animator>().SetTrigger("Damage");
}
}
}
}
}
Not surprisingly, it doesn't work and I still get crashes on some hits.
Any help would be appreciated, if possible.
Thank you!

Answer 1

From the situation you've described, I'll assume the error is being, primarily, by faulty refactoring of responsibility delegation.
More specifically, I believe it's sufficiently clear that the tutorial you've followed didn't follow single responsibility principle properly, and instead, implemented multiple responsibilities (player and AI) on a single class/file, to save time for the video, or to simplify the tutorial.
Later, when splitting the responsibilities to two scripts and two objects, you, as a beginner, was/is unaware of some details and/or pitfalls involved in reference management, and so are failing to assign references to both objects, or failing to handle missing references as the scripts are now split.
The problem arises because, if your player and AI now have different sets of scripts; one with each manager, but neither with both (unlike before, when both player and AI objects had "both"); then, either on the first or second if*manager.damage statement, the manager in question would not be found with GetComponent because it's not in that object, and upon trying to access fields/properties/methods on a null reference, the NullReferenceException would be thrown.
The solution is simply to do the proper null checks before accessing such fields/properties/methods, to guard from the exception and proceed to the second if statement if the first's manager is found to be null.
While at it, might as well cache the queries to make things better, as RetiredNinja recommended in the comments.
Code:
void OnTriggerEnter2D(Collider2D col) {
if(col.transform.root != transform.root && col.tag != "Ground" && !col.isTrigger) {
//Cache to avoid multiple queries and to simplify access
var playermanager = col.transform.GetComponent<PlayerManager>(); //One of these won't be found and will receive null instead
var aiManager = col.transform.GetComponent<AIManager>(); //One of these won't be found and will receive null instead
var animator = col.transform.root.GetComponentInChildren<Animator>();
if (aiManager != null //Null-check was missing
&& !aiManager.damage && !aiManager.blocking) { //Much nicer
aiManager.damage = true;
if(animator!=null)
animator.SetTrigger("Damage");
}
else {
if (playerManager != null && !playerManager.damage) {
playerManager.damage = true;
animator.SetTrigger("Damage");
}
}
}
}