尝试从另一个脚本调用 function 会引发 NullReferenceException 错误

Question

I am calling a function from a different script script and I am getting this error "NullReferenceException: Object reference not set to an instance of an object PlayerCollision.OnTriggerEnter (UnityEngine.Collider collision)" What do I do? This is my code.

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

public class PlayerCollision : MonoBehaviour
{

void OnTriggerEnter(Collider collision)
{ 
    
    if(collision.gameObject.CompareTag("BlackSnow"))
        {
            GameManager.Instance.blackSnowEvent(collision.gameObject);
        }
    }
}

this is the code in the script where the function is:

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

public class GameManager : MonoBehaviour
{

public ParticleSystem blackSnowParticleSystem;
private bool snowActivated = false;
public int duration = 5;


#region Singleton

private static GameManager instance;

public static GameManager Instance
{
    get
    {
        return instance;    
    }
}


void Start()
{
}

 IEnumerator wait(int arg){
    yield return new WaitForSeconds(arg);
}

public void blackSnowEvent(GameObject snowTrigger){
    if(snowActivated == false){

        snowActivated = true;
        snowTrigger.SetActive(false);

        blackSnowParticleSystem.Play();
        Debug.Log(duration);
        StartCoroutine(wait(duration));

        blackSnowParticleSystem.Stop();
       
        snowActivated = false;
    }
} 
#endregion
}

Answer 1

The member GameManager.Instance is never assigned, so it is null at runtime. You are attempting to invoke the member blackSnowEvent on a null reference. To solve, GameManager.Instance must be assigned before you attempt to invoke its members

Answer 2

The error is not because you are calling a method from a different script, it is because you are trying to pass a null value as an argument to it.

The error is because in your if statement in your void OnTriggerEnter(Collider collision) method, you are trying to check if the collided object has a tag "BlackSnow" or not. But that collider doesn't necessarily be something, it may also be null. Hence, Hence modify the if statement to this.

if(collision != null && collision.gameObject.CompareTag("BlackSnow")) {
    GameManager.Instance.blackSnowEvent(blackSnowParticleSystem, collision.gameObject);
}

In this scenario, if the compiler detects that the collision object is null, then it doesn't proceed further, therefore doesn't throw any error.