This stacktrace is what I'm getting when I try to use a button to activate and automatically select a default button in another panel, where the other panel begins as deactivated. The order of components is a UIButtonActivate to activate the other panel, then UIButtonKeys.SelectOnClick should automatically select a button in the other panel which has a UIButtonOffset attached.
SelectOnClick ends up setting the selectedObject before the object in question has run Start/Init on its components, which both throws this UnassignedReferenceException and also runs a Tween to Vector3.zero scale since it gets called before mScale is initialized.
Environment is NGUI 2.3.3h and Unity 3.5.7. It sounds similar to this issue:
http://www.tasharen.com/forum/index.php?topic=2913.msg15021#msg15021, was this fixed in 2.3.3h in the assetstore?
UnassignedReferenceException: The variable tweenTarget of 'UIButtonOffset' has not been assigned.
You probably need to assign the tweenTarget variable of the UIButtonOffset script in the inspector.
UIButtonOffset.OnHover (Boolean isOver) (at Assets/NGUI/Scripts/Interaction/UIButtonOffset.cs:61)
UnityEngine.GameObject:SendMessage(String, Object, SendMessageOptions)
UICamera:Notify(GameObject, String, Object) (at Assets/NGUI/Scripts/UI/UICamera.cs:644)
UICamera:Highlight(GameObject, Boolean) (at Assets/NGUI/Scripts/UI/UICamera.cs:631)
UICamera:set_selectedObject(GameObject) (at Assets/NGUI/Scripts/UI/UICamera.cs:359)
UIButtonKeys:OnClick() (at Assets/NGUI/Scripts/Interaction/UIButtonKeys.cs:74)
UnityEngine.GameObject:SendMessage(String, Object, SendMessageOptions)
UICamera:Notify(GameObject, String, Object) (at Assets/NGUI/Scripts/UI/UICamera.cs:644)
UICamera:ProcessTouch(Boolean, Boolean) (at Assets/NGUI/Scripts/UI/UICamera.cs:1147)
UICamera:ProcessOthers() (at Assets/NGUI/Scripts/UI/UICamera.cs:990)
UICamera:Update() (at Assets/NGUI/Scripts/UI/UICamera.cs:790)