Selectmenu throws exception when typing in a closed menu containing optgroups

If you hav a Selectmenu with optgroups, and type while it has focus, it may hit an optgroup` as a result. If this happens, you get an exception.

Fiddle: http://jsfiddle.net/coLcs5pz/1/

Just put focus on the selectmenu, and type S, and you'll get an exception in the console.

Uncaught TypeError: Cannot read property 'index' of undefined