Changes not being saved in the Inspector Editor

I'm developing a custom GUI system for Unity and I have a couple of programmed elements for now. One of then is a panel. Basically, this allow me to drag a couple of GUIZone (class that every item of my own GUI derives from) in some slots. The number of slots can be set by a int variable that basically create an array of n elements and start a loop that go from 0 to n creating an ObjectField for each of this elements.

The script I'm using is the following:

 using UnityEngine;
 using UnityEditor;
 using System.Collections.Generic;
 
 [CustomEditor(typeof(GUIZonePanel))]
 
 public class GUIZonePanelEditor : Editor {
 
  private GUIZonePanel _target;
  private int _elementsInPanel;
  
  public override void OnInspectorGUI () {
  
  GUILayout.Label("Insert the elements you want to be in this panel");
  _target = target as GUIZonePanel;
  _elementsInPanel = EditorGUILayout.IntField("Number of elements", _elementsInPanel);
  
  if(GUI.changed) {
  
  if(_elementsInPanel > _target.panelElements.Count) {
  
  for(int i = _target.panelElements.Count; i < _elementsInPanel; i++) {
  
  _target.panelElements.Add( null );
  
  }
  
  } else {
  
  for(int i = _target.panelElements.Count; i > _elementsInPanel; i--) {
  
  _target.panelElements.RemoveAt(i - 1);
  
  }
  }
  }
  
  for(int i = 0; i < _elementsInPanel; i++) {
  
  _target.panelElements[i] = (GUIZone)EditorGUILayout.ObjectField("Element " + i.ToString() ,_target.panelElements[i], typeof(GUIZone));
  
  }
  
  bool apply = GUILayout.Button( "Put elements in the panel");
  
  if(apply) {
  
  _target.applyElementsToThePanel();
  
  }
  
  bool _allign = GUILayout.Button("Allign with camera");
  
  if(_allign) {
  
  Vector3 cameraRotation = Camera.main.transform.eulerAngles;
  Vector3 cameraPosition = Camera.main.transform.position;
  Camera.main.transform.eulerAngles = new Vector3(90, 0, 0);
  Camera.main.transform.position = new Vector3(0, 10, 0);
  _target.transform.position = Vector3.zero;
  
  _target.transform.parent = Camera.main.transform;
  
  Camera.main.transform.eulerAngles = cameraRotation;
  Camera.main.transform.position = cameraPosition;
 
  }
  
  if(GUI.changed) {
  
  EditorUtility.SetDirty( _target );
  
  }
  
  }
 }

And here's the GUIZonePanel script:

 using UnityEngine;
 using System.Collections.Generic;
 
 public class GUIZonePanel : GUIZone {
  
  public List<GUIZone> panelElements;
  
  public void applyElementsToThePanel () {
  
  Vector3 resultantPosition = Vector3.zero;
  
  for(int i = 0; i < panelElements.Count; i++) {
  
  resultantPosition += panelElements[i].transform.position;
  
  }
  
  transform.position = resultantPosition/panelElements.Count;
  
  foreach(GUIZone gz in panelElements) {
  
  gz.transform.parent = this.transform;
  
  }
  }
  
  public void changeElementsVisibility (bool visible) {
  
  foreach(Renderer r in GetComponentsInChildren<Renderer>()) {
  
  r.enabled = visible;
  
  }
  
  }
  
 }

The problem is that when I deselect the object that contains the GUIPanel and select it again, every change is losted. My element number get back to 0 and this happens with almost every script where I have a list or an array and I need to attach elements to this array in the Editor.

What can I make to save this changes when the GO gets deselected?

Thanks from now!