Practice.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class Practice : MonoBehaviour {
    public float radius = 3;
    public GameObject prototype;
    public List<GameObject> r;

    public List<GameObject> Search () {
        for (int i=0; i<radius; i++) {
            GameObject created = Instantiate(prototype);
            created.transform.position =
                new Vector3(i, 0, 0) + transform.position;
            r.Add(created);
        }

        return r;
    }
}

PracticeEditor.cs

using UnityEngine;
using UnityEditor;
using UnityEditor.SceneManagement;
using System.Collections;
using System.Collections.Generic;

[CustomEditor(typeof(Practice))]
public class PracticeEditor : Editor {
    Practice Target { get => (Practice) target; }

    // void OnEnable () {
    //     Target.Search();
    // }

    // void OnDisable () {
    //     Debug.Log("unselected!");
    // }

    public override void OnInspectorGUI () {
        DrawDefaultInspector();
        bool wasPressed = GUILayout.Button("Create!");

        Undo.RecordObject(Target, "list created");
        if (wasPressed) {
            List<GameObject> created = Target.Search();
            Undo.RecordObject(Target, "changed radius");
            Target.radius++;

            foreach (GameObject c in created) {
                Undo.RegisterCreatedObjectUndo(c, "!!!");
            }
        }
    }

    void OnSceneGUI () {
        if (Target.r == null || Target.r.Count == 0) return;

        foreach (GameObject block in Target.r) {
            if (block.name == "INDESTRUCTIBLE") continue;

            bool wasPressed =
                Handles.Button(block.transform.position
                               + block.transform.forward * 0.5f
                               + block.transform.right * 0.5f,
                               Quaternion.Euler(90,0,0),
                               0.25f, 0.25f,
                               Handles.RectangleHandleCap);

            if (wasPressed) {
                Undo.RecordObject(Target, "destroyed block");
                Target.r.Remove(block);
                Undo.DestroyObjectImmediate(block);
                break;
            }
        }
    }
}