A weird NullReferenceException that would disappear in run-time when I select the gameobject in editor's hierarchy the script attach to.

Hi, I am an Architecturual Gruaduate Student of University of Wisconsin-Milwaukee. Right now I am using an personal version of unity and encounter some scripting/engine problem I can not solve myself and can not find answers from community, and my thesis project is an Unity project! I would be very appreciate if someone can help! Thank you!

Here is the YouTube link to my reproduction of the issue (100% reproducable):
Unity NullReferenceException Glitch


Here is a link to the issue I uploaded to unity.
(Case 770803) NullReferenceException on script attached to an instantiated gameobject
https://fogbugz.unity3d.com/default.asp?770803_kcs7nic90r3u2q6g

Thank you, guys!

NullReferenceException: Object reference not set to an instance of an object
SlotPanel.AddItem (Int32 id, Int32 quantity, Int32 dura) (at Assets/My Assets/System/Item/Scripts/SlotPanel.cs:514)
NEIslot.Use () (at Assets/My Assets/System/Item/Scripts/NEIslot.cs:36)
UnityEngine.Events.InvokableCall.Invoke (System.Object args) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:144)
UnityEngine.Events.InvokableCallList.Invoke (System.Object parameters) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:621)
UnityEngine.Events.UnityEventBase.Invoke (System.Object parameters) (at C:/buildslave/unity/build/Runtime/Export/UnityEvent.cs:756)
UnityEngine.Events.UnityEvent.Invoke () (at C:/buildslave/unity/build/Runtime/Export/UnityEvent_0.cs:53)
UnityEngine.UI.Button.Press () (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:35)
UnityEngine.UI.Button.OnPointerClick (UnityEngine.EventSystems.PointerEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/UI/Core/Button.cs:44)
UnityEngine.EventSystems.ExecuteEvents.Execute (IPointerClickHandler handler, UnityEngine.EventSystems.BaseEventData eventData) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:52)
UnityEngine.EventSystems.ExecuteEvents.Execute[IPointerClickHandler] (UnityEngine.GameObject target, UnityEngine.EventSystems.BaseEventData eventData, UnityEngine.EventSystems.EventFunction`1 functor) (at C:/buildslave/unity/build/Extensions/guisystem/UnityEngine.UI/EventSystem/ExecuteEvents.cs:269)
UnityEngine.EventSystems.EventSystem:Update()