iText keeps throwing NullReferenceException

I've got one issue with iText right now. I've created a WindowsForms application with C#, it uses iText for generating pdf files with orders. I'm using it in my work, just to save some time and simplify my work a bit, but it's only for me. About a week ago I've started creating another application, this time it's web app with .net core technology.

public IActionResult OnPostPrintToPDF()
    {
        byte[] pdfBytes;
        MemoryStream stream = new MemoryStream();
        PdfWriter writer = new PdfWriter(stream);
        PdfDocument pdf = new PdfDocument(writer);
        Document document = new Document(pdf);
        stream.Position = 0;
        Paragraph par;
        par = new Paragraph("Hello world");
        document.Add(par);
        document.Close();
        pdfBytes = stream.ToArray();
        return new FileContentResult(pdfBytes, "application/pdf");
    }

When I tried to use iText, there's an exception, even though code looks the same. I've tried any example source codes and nothing works with console application both .net core and framework, I keep getting the same exception (System.NullREferenceException), even in my first app where everything worked so far.

Stack trace of the exception:

System.NullReferenceException HResult=0x80004003 Message=Object reference not set to an instance of an object. Source=itext.io StackTrace: at iText.IO.Font.FontCache..cctor()

Any ideas what could happen?

The problem was related due some sort of cache. I can't really understand what, but as you can see in the code the exceptions was generate when he tried to load fonts from cache. I realized that after having tried the same code to a new project and it worked.

so I cleaned the solution, deleted bin, obj, .vs, killed IIS Express, removed and reinstalled all nuget packages then run again, magically it worked.

then I had to made only a fix to the code: instead of HtmlConverter.ConvertToDocument that generates only 15 bytes document i used HtmlConverter.ConvertToPdf to generate a full pdf.