I have a problem in Unity 2d
NullReferenceException: Object reference not set to an instance of an object RightOne.OnMouseDown () (at Assets/Scripts/RightOne.cs:13) UnityEngine.SendMouseEvents:DoSendMouseEvents(Int32)
Here's the script:
using UnityEngine;
using System.Collections;
public class RightOne : MonoBehaviour {
    private GameObject mainCube;
    void Start () {
        mainCube = GameObject.Find ("Main Cube");
    }
    void OnMouseDown () {
        if (GetComponent <Renderer> ().material.color == mainCube.GetComponent <Renderer> ().material.color)
            mainCube.GetComponent <GameCntrl> ().next = true;
        else
            mainCube.GetComponent <GameCntrl> ().lose = true;
    }
}
 
     
    