Classifieds plugin all themes page-author.php

I am receiving the error

Trying to get property of non-object in /var/www/wp-content/plugins/classifieds/ui-front/general/page-author.php on line 28

which makes sense because this bit of code should not work

if ( '' == get_option( 'permalink_structure' ) )
$cf_author_name = $_REQUEST['cf_author'];
$cf_author_name = get_query_var( 'cf_author_name' );
$query_string = 'author_name=' . $cf_author_name;
$user_data = get_user_by( 'login' ,$cf_author_name );

I looked at the docs for wordpress and it it has no option to get_user_by user name and cf_author_name is user name not log in name .