Unity UI The Dropdown will not destroy the blocker itself.

I create a dropdown and keep its option list dropped so that it will generate a blocker automatically.
But if I don't click the blocker and destroy the dropdown directly,the blocker will still stay there and blocks any other raycasts.
By now,I have to manually call its Hide function every time I destroy the dropdown.
Can't this Dropdown class just destory the blocker in its own OnDestroy?