Function picker_optional 

pub fn picker_optional<T, Sel, M>(
    selected: Sel,
    options: impl IntoPickerOptions<T, M>,
) -> Picker<T, Optional<Sel>>
where
    T: Clone + PartialEq + 'static,
    Sel: Binding<Option<T>>,

picker over an optional value: None is “nothing chosen” (no selection where the toolkit can show none), and a user’s choice writes Some(value).