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

I want to instance and destroy an object when custom editor is enabled in edit mode (ie, currently selected in scene).

However, if you press play, the created object gets leaked, despite being destroyed in the OnDisable method.

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

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

Using Unity 2018.3.0f2

 public class CreateOnClickInScene : MonoBehaviour
 {
     public GameObject CreatedObject;
 }
 
 #if UNITY_EDITOR
 [CustomEditor(typeof(CreateOnClickInScene))]
 public class CreateOnClickInSceneEditor : Editor
 {
     private CreateOnClickInScene Target { get { return (CreateOnClickInScene)target; } }
 
     private void OnEnable()
     {
         if (!Application.isPlaying)
         {
             Target.CreatedObject = new GameObject("Editor only object");
         }
     }
 
     private void OnDisable()
     {
         DestroyImmediate(Target.CreatedObject);
         Target.CreatedObject = null;
     }
 }