Destroying object immediately is not permitted during physics trigger and contact callbacks. You must use Destroy instead.
UnityEngine.Object:DestroyImmediate(Object)
NGUITools:DestroyImmediate(Object) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:441)
UIPanel:OnDisable() (at Assets/NGUI/Scripts/UI/UIPanel.cs:519)
UnityEngine.GameObject:set_active(Boolean)
NGUITools:Deactivate(Transform) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:508)
NGUITools:SetActive(GameObject, Boolean) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:523)