The problem can be solved, adding this line of code before the call of the method: System.Web.HttpUtility.HtmlDecode(string);
HttpEncoder.Current = HttpEncoder.Default;
The original answare can be found here: http://forums.xamarin.com/discussion/4805/system-nullreferenceexception-with-system-web-httputility-htmldecode#latest
And the report of the bug with a more complete explanation, can be found here: https://bugzilla.xamarin.com/show_bug.cgi?id=12565