ReadAsync issue

[A41056]

`string _zPdfFilePath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "File", "hdcc.pdf");

        var _IronOcr = new IronTesseract();
        _IronOcr.Language = OcrLanguage.Vietnamese;

        using (var _Input = new OcrInput())
        {
            _Input.AddPdfPages(_zPdfFilePath, null, null, null);
            string _zPath = "hdcc-searchable.pdf";

            var _t1 = _IronOcr.ReadAsync(_Input, null);
            var _t2 = initializeWebViewAsync();

            Task.WhenAll(_t1, _t2).ContinueWith((_t) =>
            {
                if (Disposing || IsDisposed)
                    return;

                if (_t1.IsCanceled || _t2.IsCanceled)
                    return;
                else if (_t1.Exception != null)
                    MessageBox.Show(_t1.Exception.InnerException.Message);
                else if (_t2.Exception != null)
                    MessageBox.Show(_t2.Exception.InnerException.Message);
                else
                {
                    if (InvokeRequired)
                    {
                        Invoke((MethodInvoker)(() =>
                        {
                            _t1.Result.SaveAsSearchablePdf(_zPath);

                            string _zUrl = "file:///" + Path.GetFullPath(_zPath).Replace('\\', '/');

                            if (webView != null && webView.CoreWebView2 != null)
                            {
                                webView.CoreWebView2.Navigate(_zUrl);
                            }
                        }));
                    }
                    else
                    {
                        _t1.Result.SaveAsSearchablePdf(_zPath);

                        string _zUrl = "file:///" + Path.GetFullPath(_zPath).Replace('\\', '/');

                        if (webView != null && webView.CoreWebView2 != null)
                        {
                            webView.CoreWebView2.Navigate(_zUrl);
                        }
                    }
                }
            });
        }`
        
        I want to convert from an image, scanned pdf file to a searchable pdf. It works fine with Read(), but the main thread will be blocked until the process is finished. So I'm using ReadAsync() to not block the main thread, but I have the error "Recognition of image failed". I don't know why, because the Read() run fine so it can't be an image quality problem. Please help me.