Returns the on state of widget: the state of its normal appearance other than Off.
public CosName? GetOnState(PdfWidgetAnnotation widget)
One of the field's widgets.
null when the widget has no appearance states. A normal appearance with states but none other
than Off records ButtonOnStateMissing; with several, the one equal to the value is used, else the first, with
ButtonStatesAmbiguous.
ISO 32000-2 §12.7.5.2.3 and §12.5.5.