iTextSharp issues wherein null reference exception

I am using iTextSharp with C# to fill an editable PDF. Everything was working fine, but suddenly I am getting an error on the PdfStamper line.

PdfReader pdfReader = new PdfReader(File.ReadAllBytes(pdfTemplatePath));
PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(newFile, FileMode.Create,FileAccess.Write));
AcroFields pdfFormFields = pdfStamper.AcroFields;

The error is

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=itextsharp
  StackTrace:
   at iTextSharp.text.Version.GetInstance()

I have checked the file paths and everything - that is all correct. I have also made sure there are no permission issues.

Can someone please let me what might be causing this to throw a null reference exception.

Thanks in advance.