Unity Editor - validating GameObjects

Task:

I want to create a context menu item that will work on GameObject selected in Hierarchy. This action should be available only when selected GameObject is a Prefab. I'm working on Unity 5.3.2 (tested also on 5.3.5)

Behaviour should be simillar to Select Prefab item (visible on screenshot). When object isn't connected to prefab field should be grey and not-clickable. When object is a prefab field is black and can be clicked.

Code:

File MenuItems.cs is located in Assets\Editor

 using UnityEditor;
 using UnityEngine;
 
 public static class MenuItems
 {
 
     [MenuItem("GameObject/Action on prefab", false, 14)]
     private static void MenuActionPrefab()
     {
     }
 
     [MenuItem("GameObject/Action on prefab", true, 14)]
     private static bool ValidateMenuActionPrefab()
     {
         var isPrefab = PrefabUtility.GetPrefabParent(Selection.activeGameObject) != null;
         return isPrefab;
     }
 }

Problem:

The problem is that the menu item is visible (and can be clicked) no matter if selected GameObject is a Prefab or not. I have tested code in debugger and value returned from ValidateMenuActionPrefab() is OK (`true` when I run it on Prefab, false on non-prefab gameobject).

I've read somewhere that Unity 5 have problem with validation methods but the example from UNITY EDITOR EXTENSIONS – MENU ITEMS about validation in Assets work perfectly OK.

Additional info:

I have tried to run those methods simple way using:

 [MenuItem("GameObject/Action on prefab")]

and

 [MenuItem("GameObject/Action on prefab", true)]

but in this case there is no Action on prefab in context menu. Also this item isn't visible when I try not to set priority index.

I have posted this problem also on StackOverflow Link. There is a shor discusion too.

Question:

So if this is a proper solution? Or is there another way to achieve same goal?