[Plugin: OpenInviter for WordPress] Widget error for Openinviter

I get this warning when I try to add the widget

Warning: Cannot modify header information – headers already sent by (output started at /home/ranchoco/public_html/wp-content/plugins/openinviter-for-wordpress/oi_includes/plugins/hi5.php:146) in /home/ranchoco/public_html/wp-includes/pluggable.php on line 770