pho_error.log: PHP Fatal error

Hi,

how do I get rid of these error messages?

[13-Apr-2016 23:39:30 UTC] PHP Fatal error: Class ‘WYSIJA_NL_Widget’ not found in /data/web/e48437/html/mywebsite/wp-content/plugins/wysija-newsletters/helpers/front.php on line 255

https://wordpress.org/plugins/wysija-newsletters/