Exception when generating PDF/A - Concurrent modification during iteration: Instance of '_HashSet<PdfObject<PdfDataType>>'.

[pldelattre]

Describe the bug
Crash when generating PDF/A using pdf 3.11.3

To Reproduce
Code snippet to reproduce the behavior:

Future<Uint8List> loadColorProfile() async {
  try {
    ByteData data = await rootBundle.load('assets/pdfColorProfiles/sRGB2014.icc');
    return data.buffer.asUint8List();
  } catch (e) {
    debugPrint('Error loading asset: $e');
    return Uint8List(0); 
  }
}

  Future<Uint8List> generatePdf(
      [PdfPageFormat format = PdfPageFormat.a4]) async {

    final pdf = pw.Document(
      version: PdfVersion.pdf_1_5,
      compress: true,
      metadata: PdfaRdf(
        invoiceRdf: PdfaFacturxRdf().create(),
      ).create(),
    );

    PdfaColorProfile(pdf.document, await loadColorProfile());
    
    PdfaAttachedFiles(
      pdf.document,
      {
        'factur-x.xml': '<test></test>',
      },
    );

     return pdf.save();
 )

     await generatePdf();   // EXCEPTION

     debugPrint('Never reached... Exception on previous line');

Exception doesn't occur if I remove:

    PdfaAttachedFiles(
      pdf.document,
      {
        'factur-x.xml': '<test></test>',
      },
    );

ERROR

Expected behavior
Should not crash

Screenshots

Flutter Doctor

[✓] Flutter (Channel stable, 3.24.3, on macOS 14.7.3 23H417 darwin-arm64, locale en)
[✓] Android toolchain - develop for Android devices (Android SDK version 35.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 16.2)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2024.2)
[✓] VS Code (version 1.97.0)
[✓] Connected device (4 available)
[✓] Network resources

Desktop (please complete the following information):

• iOS
• Android
• Browser
• Windows
• Linux

Smartphone (please complete the following information):

• OS: Mac OS 14.7.3
• Browser: Chrome 133.0.6943.99

[pldelattre]

@ilaurillard

[ilaurillard]

see #1820

[pldelattre]

Thanks @ilaurillard. I confirm the pull request fixes the bug!