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)