STA thread ui control.cs
public static string Convert(string rtfText)
{
Task<string> Task1 = Task.Factory.StartNewSta(() =>
{
var xamlText = ConvertRtfToXaml(rtfText);
string result = XamlToHtmlConverter.Convert(xamlText, false);
result = result.Replace("<HTML>", "").Replace("</HTML>", "").Replace("<BODY>", "").Replace("</BODY>", "").Trim();
return result;
});
Task.WaitAll(Task1);
return Task1.Result;
}