[SOLVED]I can't use Select() on my Buttons in scripts

Hi
i'm working on a menu using xbox controller

I want to select a <Button> object in a script, but get an error i can't solve

First i made a public list of buttons that i dragged and dropped in the inspector

Then later when i tryed to do this :
i got the error :

NullReferenceException: Object reference not set to an instance of an object...

[SOLVED]I can't use Select() on my Buttons in scripts