error when plugin activated

I received this error on my website when the RSVP Maker is activated

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘winwar_no_related_posts’ not found or invalid function name in /home3/hnnaps/public_html/wp-includes/class-wp-hook.php on line 298

It goes away when I de-activate. I love this plug in. Any advice on how to fix this.