How to disable a given MenuItem based on a certain condition using the isValidateFunction argument.

DisableMenuItem.cs

using UnityEditor;

public static class DisableMenuItem 
{
    [MenuItem("Examples/How to disable a custom menu item")]
    static void DisableMenuItemExample(){ }

    // Disable the menu item under a certain condition. This is where isValidateFunction comes in play, by the way.
    [MenuItem("Examples/How to disable a custom menu item", true)]
    static bool ValidateMenuItem(){ return Selection.activeGameObject != null; }
}