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.