Unity 5.5.1 - Custom editor button Undo query

Hello all,

I am making a custom editor, and one of the functionalities will be to spawn multiple prefabs at a specific positions, as well as to undo the spawn action(s). So far I have been able to spawn a single object when the "Place object" button is clicked, but now I can't get my head around on how to Undo the spawn action with an "Undo" button click. I have browsed through Unity's scripting API, many posts on this forum, as well as other, but the further I read the more confused I get.

The script I have so far is:

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 using System.Collections.Generic;
  
 [CustomEditor(typeof(ObjectSpawn))]
  
 public class ObjectSpawnEditor : Editor
 {
  
 int objectSelectionToolbar = 0;
  
 bool chooseOption = false;
 bool objectSelectionFoldout = false;
  
 public override void OnInspectorGUI()
     {
         DrawDefaultInspector();
  
         chooseOption = EditorGUILayout.Foldout(chooseOption, "Choose a part to add:");
  
         if (chooseOption)
         {
  
             //Check Station selection button
  
             EditorGUILayout.BeginVertical();
  
             objectSelectionFoldout = GUILayout.Toggle(objectSelectionFoldout, "" + (objectSelectionFoldout ? "▼ Object selection ▼" : "► Object selection ◄"), "Button", GUILayout.MaxWidth(Screen.width), GUILayout.Height(25));
  
             if (objectSelectionFoldout)
             {
  
                 GUILayout.Space(5);     //Space before a text box
  
                 GUILayout.Box("Select lenght:");
  
                 GUILayout.Space(5);     //Space after a text box and before a toolbox
  
                 string[] stationSelectionToolbarOptions = new string[] { "Object A", "Object B", "ObjectC" };
  
                 objectSelectionToolbar = GUILayout.Toolbar(objectSelectionToolbar, stationSelectionToolbarOptions, GUILayout.MinWidth(Screen.width), GUILayout.Height(50));
  
                 GUILayout.Space(5);
  
                 switch (objectSelectionToolbar)
                 {
                     case 0:
  
                         GUILayout.BeginHorizontal();
  
                         if (GUILayout.Button("Place object", GUILayout.Height(25)))
                         {
                        
                             ObjectSpawn myScript = (ObjectSpawn)target;
                             myScript.AttachObject();
                      
                         }
  
                         if (GUILayout.Button("Undo", GUILayout.Height(25)))
                         {
                        
  
                         }
                         GUILayout.EndHorizontal();
  
                         GUILayout.Space(5);
  
                         GUILayout.Box("Information:", GUILayout.MaxWidth(Screen.width), GUILayout.Height(50));
  
                         GUILayout.Space(5);
  
                         break;
                 }
  
             }
  
             EditorGUILayout.EndVertical();
          }
       }
 }

And the Monobehaviour script to attach the object, as well as to parent the spawned object to the object that has the script attached to:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
  
 public class ObjectSpawn : MonoBehaviour {
  
     public void AttachObject()
     {
         GameObject Object_A = Resources.Load("Prefab") as GameObject;
         Instantiate(Object_A, this.transform.position, this.transform.rotation, this.transform);
     }
 }

In Unity it is showing as bellow:

"Start Point" is just an empty game object that has the script "ObjectSpawn" attached to.

Could anyone provide any ideas or steps for the solution? Any suggestions are welcome! Thanks