When I "Pause" game, it return to Main Menu

I have 4 scenes: - Scene 0: Main Menu. - Scene 1 to 3: Level 1 to 3. I follow a Youtube tutorial to create a "Pause Menu" with "Resume", "Main Menu" button in each level with Escape Key, when I press the button, the Pause Menu appear and immediately the Main Menu scene is loaded. How can I fix it ? Thanks.

Here is my Pause Menu code:

 using UnityEngine;
 using System.Collections;
 
 public class PauseMenu : MonoBehaviour {
     public GUISkin mySkin;
     private Rect windowRect;
     private bool paused = false, waited = true;
 
     void Start(){
         windowRect = new Rect (Screen.width / 2 - 100, Screen.height / 2 - 100, 200, 200);
     }
 
     void Update(){
         if(waited)
             if (Input.GetKey (KeyCode.Escape)) {
                 if (paused) paused = false;
                 else paused = true;
                 waited = false;
                 Invoke("waiting", 0.3f);
             }
         if (paused)
                         Time.timeScale = 0;
                 else
                         Time.timeScale = 1;
     }
 
     void waiting(){
         waited = true;
     }
 
     void OnGUI(){
         if (paused)
                         windowRect = GUI.Window (0, windowRect, windowFunc, "Pause Menu");
     }
 
     void windowFunc(int id){
         if (GUILayout.Button ("Resume"))
                         paused = false;
         GUILayout.BeginHorizontal();
         if (GUILayout.Button ("Option")) {
                 }
         if (GUILayout.Button ("Quit")) {
                 }
         GUILayout.EndHorizontal();
     }
 }