Notices are trown

For prevent notices you should check some variables on style_url_enqueued
Function modified (style with php5 compatibility)

  public function style_url_enqueued($url){
    global $wp_styles;
    foreach($wp_styles->queue as $wps_name){
      $wps = (isset($wp_styles->registered[$wps_name]))? $wp_styles->registered[$wps_name] : "";
      if(isset($wps->src) && $wps->src == $url){
        return $wps;
      }
    }
    return false;