コントロール ID から既存の状態オブジェクトを取得します
This will return a recycled state object that is unique for controlID.
If the state object has not been created by calling GetStateObject then it
cannot be accessed using QueryStateObject. A call into QueryStateObject
with the state object not created is invalid. A null may be returned, but is not
guaranteed. An exception may happen instead.
See Also: GUIUtility.GetStateObject.