Error at building by pdf 3.11.2 and pdf_crypto 3.3.9

[chrisoelt]

Here is the Error Code:
Pub/Cache/hosted/pub.nfet.net%2547MzQzMGVmYWYxMzF%2547/pdf_crypto-3.3.9/lib/src/parser/form_field.dart:75:27: Error: The type 'Set?' of the getter 'PdfParsedFormField.flags' is not a subtype of the type 'Set' of the inherited setter 'PdfAnnotBase.flags'.

• 'Set' is from 'dart:core'.
• 'PdfAnnotFlags' is from 'package:pdf/src/pdf/obj/annotation.dart' ('/Local/Pub/Cache/hosted/pub.dev/pdf-3.11.2/lib/src/pdf/obj/annotation.dart').
  Set? get flags => throw UnimplementedError();
  ^^^^^