iTextSharp.tool.xml.exceptions.RuntimeWorkerException: 'Inva

I'm facing above metioned errro where my controller code is given below:

StringReader sr = new StringReader(GridHtml);

Document pdfDoc = new Document(PageSize.A4, 80, 50, 30, 65);

// Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 100f, 0f);

PdfWriter writer = PdfWriter.GetInstance(pdfDoc, stream);

Rectangle rect = new Rectangle(100, 100, 100, 100);

RadioCheckField checkbox = new RadioCheckField(writer, rect, "bhjabsdf", "on");

pdfDoc.Open();

checkbox = new RadioCheckField(writer, rect, "abc", "on");

checkbox.CheckType = RadioCheckField.TYPE_CHECK;

PdfFormField field = checkbox.CheckField;

XMLWorkerHelper.GetInstance().ParseXHtml(writer, pdfDoc, sr);

pdfDoc.Close();

System.Diagnostics.Process.Start("C:\\Users\\Mudassar Iqbal\\Downloads\\Grid.pdf");

return File(stream.ToArray(), "application/pdf", "Grid.pdf");

What shoud i do to remove or solve this error and same error i'm facing when i've to convert check boxes and radio buttons into pdf