error in my wp debug.log

Hello,

I'm getting this error in my wp debug.log:

[03-Mar-2017 21:32:01 UTC] PHP Notice: Undefined index: HTTP_USER_AGENT in /var/www/eistnaflug.is/wp-content/plugins/wp-defender/app/module/ip-lockout-module/controller/lockout-controller.php on line 115
[04-Mar-2017 03:03:10 UTC] PHP Notice: Trying to get property of non-object in /var/www/eistnaflug.is/wp-content/plugins/wpmu-dev-seo/includes/tools/onpage.php on line 514
[04-Mar-2017 03:08:08 UTC] PHP Notice: Trying to get property of non-object in /var/www/eistnaflug.is/wp-content/plugins/wpmu-dev-seo/includes/tools/onpage.php on line 514
[04-Mar-2017 09:15:02 UTC] PHP Notice: Undefined index: HTTP_USER_AGENT in /var/www/eistnaflug.is/wp-content/plugins/wp-defender/app/module/ip-lockout-module/controller/lockout-controller.php on line 115
[04-Mar-2017 09:15:02 UTC] PHP Notice: Undefined index: HTTP_USER_AGENT in /var/www/eistnaflug.is/wp-content/plugins/wp-defender/app/module/ip-lockout-module/controller/lockout-controller.php on line 115

How to fix ?