Gets the pop-up annotation for entering or editing the text (Popup, PDF 1.3), or null.
public PdfPopupAnnotation? Popup { get; }
ISO 32000-2 §12.5.6.2, Table 172. The same instance as on the page. When the pop-up's own Parent names another
annotation, PopupLinkMismatch is recorded: this entry says which pop-up belongs to this annotation, the pop-up's
Parent which annotation the pop-up belongs to.