SearchResults Widget problem

Hi,

I found a solution for filtering posts using QueryString, but I can't set UrlEvaluationMode to "QueryString". When I set this field to "QueryString" and save changes, then it automatically sets back this field to "UrlPath". I tried to set it even in blank page, but also nothing had changed.

Link to solution for filtering posts I found: www.sitefinity.com/.../url_filtering_in_built-in_content_widgets

Thanks in advance,

Michał