Gets the annotation this pop-up belongs to: its Parent, or, when absent, the markup annotation of the same page whose Popup names it; null when there is none.
public PdfAnnotation? Parent { get; }
ISO 32000-2 §12.5.6.14, Table 186. A parent that is not a markup annotation records PopupParentInvalid; one whose Popup names another pop-up, PopupLinkMismatch.