Object created in CustomEditor is leaked into scene when play is pressed

I'm attempting to instance an object when a custom editor is enabled, and destroy that object when custom editor is disabled.

However, if you press play while custom editor is enabled, the created object does not get destroyed. It remains in the scene after you stop playing. OnDisable is called just before play starts, so it seems like a bug that the object is leaked into my scene.

Gif demonstration of issue: https://i.imgur.com/EZOXOmx.gif

Minimal project reproducing the issue: https://github.com/paulmriordan/unity-custom-editor-instance-disable-bug

Steps to reproduce:

• Select object with CreateOnClickInScene
• Note object is created in scene
• Start playing in editor
• Stop playing in editor

Outcome:

• Created object remains in the scene

Expected behaviour:

• Created object should have been destroyed and no longer in scene

Using Unity 2018.3.0f2

Script:

Code (CSharp):

1. public class CreateOnClickInScene : MonoBehaviour
2. {
3.      public GameObject CreatedObject;
4. }
5. #if UNITY_EDITOR
6. [CustomEditor(typeof(CreateOnClickInScene))]
7. public class CreateOnClickInSceneEditor : Editor
8. {
9.      private CreateOnClickInScene Target { get { return (CreateOnClickInScene)target; } }
10.      private void OnEnable()
11.      {
12.          if (!Application.isPlaying)
13.          {
14.              Target.CreatedObject = new GameObject("Editor only object");
15.          }
16.      }
17.      private void OnDisable()
18.      {
19.          DestroyImmediate(Target.CreatedObject);
20.          Target.CreatedObject = null;
21.      }
22. }

Anyone know if this is an open bug, or is there alternative way to do this?