How do you fix NullReferenceException?

Questions & Answers
1

When the Pong Cube hits the “Goal” wall it should add a point to the proper side. However I keep getting “NullReferenceException: Object reference not set to an instance of an object Goal.GetPoint () (at Assets/Scripts/Goal.cs:12)”

Here is my code :

Ball

using UnityEngine;
using System.Collections;

public class Ball : MonoBehaviour 
{
	//The speed the ball starts with
	public float StartSpeed = 5f;

	//The maximum speed of the ball
	public float MaxSpeed = 20f;

	//How much faster the ball gets with each bounce
	public float SpeedIncrease = 0.25f;

	//Current speed of the ball
	public float currentSpeed;

	//Current direction of the ball
	public Vector2 currentDir;

	//Whether or not the ball is resetting
	private bool resetting = false;

	public Goal goal;

	void Start () 
	{
		//initialize starting speed
		currentSpeed = StartSpeed;

		//initialize starting direction
		currentDir = Random.insideUnitCircle.normalized;
	}

	void Update () 
	{
		//don't move the ball if it's resetting
		if (resetting)
			return;

		//move the ball in the current direction
		Vector2 moveDir = currentDir * currentSpeed * Time.deltaTime;
		transform.Translate(new Vector3(moveDir.x, 0f, moveDir.y));
	}

	void OnTriggerEnter(Collider other)
	{
		if (other.tag == "Boundry") 
		{
			//vertical boundary, reverse y direction
			currentDir.y *= -1;
		}
		else if(other.tag == "Player")
		{
			//Player paddle, reverse x direction
			currentDir.x *= -1;
		}
		else if(other.tag == "Goal")
		{

			//reset the ball
			StartCoroutine(resetBall());
			//inform goal of the score
			Debug.Log ("before other message sent");
			other.SendMessage("GetPoint", SendMessageOptions.DontRequireReceiver);
			Debug.Log ("After other message sent");
		}

		//increase speed
		currentSpeed += SpeedIncrease;

		//clamp speed to maximum
		currentSpeed = Mathf.Clamp (currentSpeed, StartSpeed, MaxSpeed);
	}

	IEnumerator resetBall()
	{
		//Reset position, speed and direction
		resetting = true;
		transform.position = Vector3.zero;

		currentDir = Vector3.zero;
		currentSpeed = 0f;

		//wait for 3 seconds before starting the round
		yield return new WaitForSeconds (3f);

		Start ();

		resetting = false;
	}
}

Goal

using UnityEngine;
using System.Collections;
public class Goal : MonoBehaviour
{
	// the player who gets a point for this goal, 1 or 2
	public int Player = 2;
	// the Scorekeeper
	public Scorekeeper scorekeeper;
	public void GetPoint()
	{
		// when the ball collides with this goal, give the player a point
		scorekeeper.AddScore(Player);
	}
}

Scorekeeper

using UnityEngine;
using System.Collections;
public class Scorekeeper : MonoBehaviour
{
	// the maximum score a player can reach
	public int ScoreLimit = 10;
	// the display test for player 1's score
	public TextMesh Player1ScoreDisplay;
	// the display text for player 2's score
	public TextMesh Player2ScoreDisplay;
	// Player 1's score
	private int p1Score = 0;
	// Player 2's score
	private int p2Score = 0;
	// give the appropriate player a point
	public void AddScore( int player )
	{
		// player 1
		if( player == 1 )
		{
			p1Score++;
		}
		// player 2
		else if( player == 2 )
		{
			p2Score++;
		}

		// check if either player reached the score limit
		if( p1Score >= ScoreLimit || p2Score >= ScoreLimit )
		{
			// player 1 has a better score than player 2
			if( p1Score > p2Score )
				Debug.Log( "Player 1 wins" );
			// player 2 has a better score than player 1
			if( p2Score > p1Score )
				Debug.Log( "Player 2 wins" );
			// both players have the same score - tie
			else
				Debug.Log( "Players are tied" );
			// reset scores and start over
			p1Score = 0;
			p2Score = 0;
		}
		// display each player's score
		Player1ScoreDisplay.text = p1Score.ToString();
		Player2ScoreDisplay.text = p2Score.ToString();
	}
}
2

You are not initializing ‘scorekeeper’. Assuming the Scorekeeper script is attached to some object in the scene, you can initialize it as follows:

using UnityEngine;
using System.Collections;
public class Goal : MonoBehaviour
{
    // the player who gets a point for this goal, 1 or 2
    public int Player = 2;
    // the Scorekeeper
    public Scorekeeper scorekeeper;
    
    public void Start() {
        scorekeeper = GameObject.Find("ScoreKeeperObject").GetComponent<Scorekeeper>();
    }

    public void GetPoint()
    {
       // when the ball collides with this goal, give the player a point
       scorekeeper.AddScore(Player);
    }
}

‘ScoreKeeperObject’ is whatever name you’ve given the game object that has the Scorekeeper script.

More info on accessing other game objects:

http://docs.unity3d.com/412/Documentation/ScriptReference/index.Accessing_Other_Game_Objects.html