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

Questions & Answers
1

hello there , i making a Question and answer game in unity , but i get this problem whenever i try to check for the correct response , i have three scripts, the first one game_uGUI which basically set the questions and answers from the files , the second is call_win and the third is call_lose.

i have four buttons in the scene and everything is attached to its place scripts and everything

but one i test, it gives back :
NullReferenceException: Object reference not set to an instance of an object
game_uGUI.button_two_logic () (at Assets/mobile game menu kit/script/game_scene/game_uGUI.cs:414)

maybe my approach here is causing the problem as i am calling onMousDown() from both call_win& call_lose.
this is game_uGUI script
public void button_four_logic()
{
if (response_4.text.CompareTo(correct_rsp) == 0)
{
call_win.OnMouseDown();
}
else
call_lose.OnMouseDown();
}

	public void button_one_logic()
	{
		if (response_1.text.CompareTo(correct_rsp) == 0)
		{
			call_win.OnMouseDown();
		}
		else
			call_lose.OnMouseDown();

	}
	public void button_two_logic()
	{
		if (response_2.text.CompareTo(correct_rsp) == 0)
		
			call_win.OnMouseDown();
		
		else
			call_lose.OnMouseDown();
	}

	public void button_three_logic()
	{
		if (response_3.text.CompareTo(correct_rsp) == 0)
		{
			call_win.OnMouseDown();
		}
		else
			call_lose.OnMouseDown();
	}

this is call_win script

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class call_win : MonoBehaviour {

	public int star_score;
	game_uGUI my_game_uGUI;
	public int score;
	public Text pscore;
	void Start()
	{
		my_game_uGUI = GameObject.FindGameObjectWithTag("_gui_").GetComponent<game_uGUI>();

	}

	public	void OnMouseDown ()
	{
		if (!game_uGUI.in_pause)
			{
			my_game_uGUI.star_number = star_score;
			pscore.text = "100";
			my_game_uGUI.Victory();
			}
	}
}

this is the call_lose script:
using UnityEngine;
using System.Collections;

public class call_lose : MonoBehaviour {

	game_uGUI my_game_uGUI;

	void Start()
	{
		my_game_uGUI = GameObject.FindGameObjectWithTag("_gui_").GetComponent<game_uGUI>();
	}

	public	void OnMouseDown ()
	{
		if (!game_uGUI.in_pause)
			{
			my_game_uGUI.Defeat();
			}
	}
}
2

your error means that whatever gameobject variable your script is reffering to is empty. you are making your script try to check a gameobject variable that hasn’t been filled yet.