NullReferenceException: Object reference not set to an instance of an object

Questions & Answers
1

I am making a simple 2D game and I made a projectile that flies towards the player and calls a method from the player script when it touches it.
The strangest thing about this error is that it worked before. A few hours ago it has worked perfectly fine and all of a sudden it is throwing this exception at me, but I didn’t even touch the script attached to my projectile.

projectile script:
using UnityEngine;
using System.Collections;

public class Bullet : MonoBehaviour {

    public Player player;

    void Start ()
    {
        player = GameObject.FindGameObjectWithTag("Player").GetComponent<Player>();
    }

    void OnTriggerEnter2D(Collider2D col)
    {
        if (col.CompareTag("Player"))
        {
            player.Damage();
        }
    }
}

The method I am calling from my Player script:

public void Damage()
    {
        tookDamage = true;
        anim.SetBool("PlayerHit", true);
        HP -= 1;

    }

The error:
NullReferenceException: Object reference not set to an instance of an object
Bullet.OnTriggerEnter2D (UnityEngine.Collider2D col) (at Assets/Bullet.cs:17)

2

It looks like player is null, are you sure it is filled in the inspector?

To be sure you’d better do something like this:

void OnTriggerEnter2D(Collider2D col)
        {
            if (col.CompareTag("Player"))
            {
                Player player = col.GetComponent<Player>();
                if (player != null) player.Damage();
            }
        }