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

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();
             }
     }
 }