List of custom classes in MonoBehaviour's Data gets lost after Re-Opening Unity!!!

I have a trouble with List of custom classes in MonoBehaviour missing data after I reopen unity... I have tried set it to dirty, but it didn't work....

I have a list of custom class, in the monobehaviour. Then i have an editor code of it that Add and remove that list with button clicks. The list works perfectly fine in run-time and editor time. HOWEVER, once i save and close the project, then re-open it.. the LIST BECOMES EMPTY

/////////////////////////////////////////////////////////////////////////////

 using System;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class MYTEST : MonoBehaviour
 {
     [SerializeField]
     public List<test> tList;
 
     [Serializable]
     public class test
     {
         public int t;
     }
 }
 
 ///////////////////////////////////////////////////////////////////
 
 using System.Collections.Generic;
 using UnityEditor;
 using UnityEngine;
 
 [CustomEditor(typeof(MYTEST)), CanEditMultipleObjects]
 public class MYTESTEDITOR : Editor
 {
     private MYTEST myTest;
     private SerializedObject sb;
 
     public void OnEnable()
     {
         myTest = (MYTEST)target;
         sb = new SerializedObject(target);
     }
 
     public override void OnInspectorGUI()
     {
         sb.Update();
 
         DrawDefaultInspector();
 
         EditorUtility.SetDirty(myTest);
 
         if (GUILayout.Button("ADD"))
         {
             if (myTest.tList == null)
             {
                 myTest.tList = new List<MYTEST.test>();
             }
             myTest.tList.Add(new MYTEST.test());
         }
 
         if (GUILayout.Button("RMV"))
         {
             myTest.tList.RemoveAt(myTest.tList.Count - 1);
         }
 
         sb.ApplyModifiedProperties();
     }
 }