Why does my Unity Serialization not work? Do i miss something?

Like my Question, what am i doing wrong? I feel like i read every post so far but really nothing helped...

 using UnityEngine;
 using UnityEditor;
 using System;
 using System.Collections;
 using System.Collections.Generic;
 
 [Serializable]
 public class QuestEditor : EditorWindow 
 {
     //[SerializeField]
     //public List<QuestEditorNode> Nodes;
 
     [SerializeField]
     private int x;
 
     void OnEnable()
     {
         Debug.Log("OnEnable " + x);
         x = 42;
     }
 
     void OnDisable()
     {
         Debug.Log("OnDisable");
         x = 66;
 
     }
 
     #region Unity Menu Tab
     [MenuItem("Quest Editor/Show Window")]
     public static void ShowWindow()
     {
         EditorWindow.GetWindow<QuestEditor>();
     }
     #endregion
 }

i compressed it hard now. like you see in the first Line my attempt is to save a list of Nodes. But since this x saving doesn't even work, i don't care about my Nodes for now...

My Code should do nothing more to just save this x when my EditorWindow is closed. I read that this should be done by just adding [Serializable] and [SerializeField] to the class/variables...