'Invalid URI: the format of the URI could not be determined.' error in actionresult method

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C#

ASP.NET-Core

.NET6

Hello, in my .NET6 MVC app I have created an endpoint which accepts two strings, one containing data which will be transformed into a file and and second one which takes a type of file which will we want to create (which is "pdf").

[HttpPost]
     public ActionResult DownloadFile([FromForm] string data, [FromForm] string fileType)
     {
         try
         {
             if (!string.IsNullOrWhiteSpace(data))
             {
                 return GenerateReportDocument(data, fileType);
             }
         }
         catch (Exception ex)
         {
             logger.LogError(ex, $"Unexpected error occured in {nameof(DownloadFile)}.");
             return NoContent();
         }

         return NoContent();
     }

Then data is taken into GenerateReportDocument method:

private ActionResult GenerateReportDocument(string data, string fileType)
    {
        var specificationString = specificationGeneratorService.GenerateSpecificationString(JObject.Parse(data));

        logger.LogWarning($"Check images in specificationString: '{specificationString}'");

        if (string.IsNullOrWhiteSpace(specificationString))
        {
            specificationString = "<p></p>";
        }

        var reportGenerator = generateReportDocuments.SingleOrDefault(r => r.FileType.ToLower().Equals(fileType.ToLower()));

        if (reportGenerator != null)
        {
            reportGenerator.GenerateReportDocument(SpecificationFileName, specificationString);
        }

        return NoContent();
    }

Which then is supposed to be taken into third method:

public HttpContent GenerateReportDocument(string fileName, string specificationString)
{
    var requestContent = new StringContent(JsonConvert.SerializeObject(new { Html = specificationString }));
    requestContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");

    var httpResponse = Flurl.Url.Combine(_abcPdfOptions.PdfConverterUrl, "pdf/convertfromhtmltopdf")
            .PostAsync(requestContent).GetAwaiter().GetResult();

    HttpContent httpContent = httpResponse.Content;

    httpContent.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
    {
        FileName = $"{fileName}.{FileExt}",
    };

    httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");

    return httpContent;
}

But when the line

reportGenerator.GenerateReportDocument(SpecificationFileName, specificationString);

in the second method is executed there appears an error "System.UriFormatException: 'Invalid URI: The format of the URI could not be determined.'" Which causes the request to crash.

What I have tried:

I have tried editing my code and changing the methods into IAtionResult or HttpResponse, but results still remains the same. Also I have tried adding extra validation to make sure that data sent into endpoint is correct, but with no success - the error remains.