Page in blank after saving

Hello

First of all, thanks for the plugin.

After installing it and configuring the plugin, I′m having the following error, that ocurs after every time I save info to database (ie: saving or update a post, uploading an image, etc.):

Warning: Cannot modify header information – headers already sent by (output started at /var/www/test.humming.cl/t/TrebolIntranet2/wp-content/plugins/birthdays-widget/class-birthdays-widget.php:1) in /var/www/test.humming.cl/t/TrebolIntranet2/wp-includes/pluggable.php on line 1195

Thanks

https://www.ads-software.com/plugins/birthdays-widget/