fix(notification): correct undefined variable

[sc979]

Description

Correct undefined variable in /var/log/httpd/error_log :

[Wed Apr 28 07:23:17.497905 2021] [:error] [pid 317] [client 172.17.0.1:33300] PHP Notice:  Undefined variable: centreon in /usr/share/centreon/www/include/monitoring/status/Notifications/notifications.php on line 117, referer: http://172.17.0.2/centreon/main.php
[Wed Apr 28 07:23:17.497937 2021] [:error] [pid 317] [client 172.17.0.1:33300] PHP Notice:  Trying to get property of non-object in /usr/share/centreon/www/include/monitoring/status/Notifications/notifications.php on line 117, referer: http://172.17.0.2/centreon/main.php
[Wed Apr 28 07:23:17.497945 2021] [:error] [pid 317] [client 172.17.0.1:33300] PHP Fatal error:  Call to a member function get_id() on a non-object in /usr/share/centreon/www/include/monitoring/status/Notifications/notifications.php on line 117, referer: http://172.17.0.2/centreon/main.php

Fixes # (issue)

Type of change

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software
• Updating documentation (missing information, typo...)

Target serie

• 2.8.x
• 19.10.x
• 20.04.x
• 20.10.x
• 21.04.x (master)