Mine

static char *
retrive_screenshot_format(){

        gchar *img_format = NULL;
        GtkComboBox *combo_box;
        ScreenshotDialog *dialog;

        dialog = g_new0 (ScreenshotDialog, 1);
        dialog->xml = glade_xml_new (GLADEDIR "/gnome-screenshot.glade", NULL, NULL);

        combo_box = glade_xml_get_widget (dialog->xml, "combo_box");

        img_format =  gtk_combo_box_get_active_text(combo_box);

         g_printf("before ' %s' ", img_format);
        if (img_format == NULL)
                img_format = "png";

                g_printf("after '%s'", img_format);

        return img_format;


}