Merge error in PDFs with Outline metadata

[clown]

Under some conditions, merging PDFs causes the following exception. According to this, we suspect that the Outline metadata causes the exception, but we don't know how to reproduce the condition. Please let me know if you have any information.

Cube.Pdf.Editor.RibbonViewModel - System.NullReferenceException: 
   Location iText.Kernel.Pdf.PdfCatalog.ConstructOutlines(PdfDictionary outlineRoot, IDictionary`2 names)
   Location iText.Kernel.Pdf.PdfCatalog.GetOutlines(Boolean updateOutlines)
   Location iText.Kernel.Pdf.PdfPage.GetOutlines(Boolean updateOutlines)
   Location iText.Kernel.Pdf.PdfDocument.CopyPagesTo(IList`1 pagesToCopy, PdfDocument toDocument, Int32 insertBeforePage, IPdfPageExtraCopier copier)
   Location iText.Kernel.Pdf.PdfDocument.CopyPagesTo(IList`1 pagesToCopy, PdfDocument toDocument, IPdfPageExtraCopier copier)
   Location iText.Kernel.Utils.PdfMerger.Merge(PdfDocument from, IList`1 pages)
   Location Cube.Pdf.Itext.DocumentWriter.OnSave(String path)