Currently this is not a feature in the widget you will have to modify the plugin. All you have to do is move code up and down.
Look for the following code
if( empty( $noviewed )) {
$output .= '<ul id="pop-widget-viewed-'.$this->number.'">';
$output .= $this->functions->get_most_viewed( $instance );
$output .= '</ul>';
}
and move it above this line
//most comments
That is it.