How to convert HTML to Docx using .net core, c# for free

Question

I need to convert HTML to DOCX or PDF to DOCX for free, but i can not to find any fresh answers. I tried this code:

 public static byte[] HtmlToWord(String html)
    {
        const string filename = "test.docx";
        if (File.Exists(filename)) File.Delete(filename);

        using (MemoryStream generatedDocument = new MemoryStream())
        {
            using (WordprocessingDocument package = WordprocessingDocument.Create(
                   generatedDocument, WordprocessingDocumentType.Document))
            {
                MainDocumentPart mainPart = package.MainDocumentPart;
                if (mainPart == null)
                {
                    mainPart = package.AddMainDocumentPart();
                    new Document(new Body()).Save(mainPart);
                }

                HtmlConverter converter = new HtmlConverter(mainPart);
                Body body = mainPart.Document.Body;

                var paragraphs = converter.Parse(html);
                for (int i = 0; i < paragraphs.Count; i++)
                {
                    body.Append(paragraphs[i]);
                }

                mainPart.Document.Save();
            }

            return generatedDocument.ToArray();
        }

But in this code I am getting the null reference exception var paragraphs = converter.Parse (html); can someone help solve the current error or advise other ways to convert

Update 1

This html throw null reference exception. I generate it of razor view.

Code to generate html of razor view

 private readonly IRazorViewEngine _razorViewEngine;
        private readonly ITempDataProvider _tempDataProvider;
        private readonly IServiceProvider _serviceProvider;

        public ViewRenderService(IRazorViewEngine razorViewEngine,
            ITempDataProvider tempDataProvider,
            IServiceProvider serviceProvider)
        {
            _razorViewEngine = razorViewEngine;
            _tempDataProvider = tempDataProvider;
            _serviceProvider = serviceProvider;
        }

        public async Task<string> RenderToStringAsync(string viewName, object model)
        {
            var httpContext = new DefaultHttpContext { RequestServices = _serviceProvider };
            var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());

            using (var sw = new StringWriter())
            {
                var viewResult = _razorViewEngine.FindView(actionContext, viewName, false);

                if (viewResult.View == null)
                {
                    throw new ArgumentNullException($"{viewName} does not match any available view");
                }

                var viewDictionary = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary())
                {
                    Model = model
                };

                var viewContext = new ViewContext(
                    actionContext,
                    viewResult.View,
                    viewDictionary,
                    new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
                    sw,
                    new HtmlHelperOptions()
                );

                await viewResult.View.RenderAsync(viewContext);
                return sw.ToString();
            }
        }