How to use a custom editor window to edit the variables of a manager script ?

Hello everyone. This is my first post ! I am currently trying to build a custom editor window to edit the settings of a sound manager ( using wwise, but it doesnt matter here). My sound manager has public variables, such as strings, PhysicMaterials etc.... It is build as a singleton. Here is juste a simple version with just the Sound Event to play on start (this would call lots of thing in Wwise, reseting parameters etc...;)

 using UnityEngine;
 using System.Collections;
 
 public class SND_GlobalSoundSet : MonoBehaviour {
 
     public static SND_GlobalSoundSet instance { get; private set;}
     public string SoundEventStartGame;
     
     void Awake ()
     { 
     
          instance = this ;
     
     }
     
 
     void OnStart (); 
     
     {
         AkSoundEngine.PostEvent (SoundEventStartGame, GameObject.Find ("DefaultSoundEmitter");
         
     }
 
 
 }

I am accessing it for instance to play global sound events (refered as strings), to call a method returning a sound switch value for wwise as a string with a PhysicMaterial paramaters, this kind of this. It s all working good exept that i need to put the manager script on a prefab to edit the variable in the inspector.
I would like to do this more properly and have a custom editor window displaying all the editable variables of my manager.
I have been trying this

 using UnityEngine;
 using UnityEditor;
 public class SoundSettings : EditorWindow {
     string eventstartgame = SND_GlobalSoundSet.instance.SoundEventStartGame;
 
     [MenuItem ("Window/Sound Settings")]
     static void Init () {
     
         SoundSettings window = (SoundSettings)EditorWindow.GetWindow (typeof (SoundSettings));
         window.Show();
     }
     
     void OnGUI () {
     
     
         GUILayout.Label ("Engine Events", EditorStyles.boldLabel);
         
         eventstartgame = EditorGUILayout.TextField ("SoundEventStartGame", eventstartgame);
         
     }
 }

It doesnt seem to work, something to do with a missing reference of a object. I m not sure I m doing this properly. I m just trying to edit the variables of my manager into this custom editor window. Anybody who could help, it would be really appreciated. Thanks