Where is TextFormFieldBuilder? - itext7

Question

The code below, which can be found in the last example here (https://kb.itextpdf.com/home/it7kb/examples/creating-form-fields) uses a class called TextFormFieldBuilder. This class doesn't seem to exist in the API though (at least not for c#). I just downloaded the latest nuget package, and the link has "it7kb" so I assume this documentation is for itext 7.
What am I missing? What do I need to do to make the example work?
namespace iText.Samples.Sandbox.Events
{
public class GenericFields
{
public static readonly String DEST = "results/sandbox/events/generic_fields.pdf";
public static void Main(String[] args)
{
FileInfo file = new FileInfo(DEST);
file.Directory.Create();
new GenericFields().ManipulatePdf(DEST);
}
protected void ManipulatePdf(String dest)
{
PdfDocument pdfDoc = new PdfDocument(new PdfWriter(dest));
Document doc = new Document(pdfDoc);
Paragraph p = new Paragraph();
p.Add("The Effective Date is ");
Text day = new Text(" ");
day.SetNextRenderer(new FieldTextRenderer(day, "day"));
p.Add(day);
p.Add(" day of ");
Text month = new Text(" ");
month.SetNextRenderer(new FieldTextRenderer(month, "month"));
p.Add(month);
p.Add(", ");
Text year = new Text(" ");
year.SetNextRenderer(new FieldTextRenderer(year, "year"));
p.Add(year);
p.Add(" that this will begin.");
doc.Add(p);
doc.Close();
}
private class FieldTextRenderer : TextRenderer
{
protected String fieldName;
public FieldTextRenderer(Text textElement, String fieldName) : base(textElement)
{
this.fieldName = fieldName;
}
// If renderer overflows on the next area, iText uses getNextRender() method to create a renderer for the overflow part.
// If getNextRenderer isn't overriden, the default method will be used and thus a default rather than custom
// renderer will be created
public override IRenderer GetNextRenderer()
{
return new FieldTextRenderer((Text) modelElement, fieldName);
}
public override void Draw(DrawContext drawContext)
{
PdfTextFormField field = new TextFormFieldBuilder(drawContext.GetDocument(), fieldName)
.SetWidgetRectangle(GetOccupiedAreaBBox()).CreateText();
PdfAcroForm.GetAcroForm(drawContext.GetDocument(), true)
.AddField(field);
}
}
}
}
EDIT: I tried the following as it seems to be equivalent logic, but when I run it I get a null reference object on the following line. Specifically, the null reference error happens on the .AddField(field) method call on the last line of the Draw method, but on inspection there is nothing that is null on that line so the error must be coming within that method so I can't tell what the issue is.
PdfTextFormField field = PdfTextFormField.CreateText(drawContext.GetDocument(), GetOccupiedAreaBBox());