PHP Warning: Undefined variable $new_lines in /usr/local/ispconfig/server/lib/classes/file.inc.php on line 126

When creating a new shell user:

DEBUG [shelluser base plugin.inc:432] - ssh-rsa setup shelluser_base
PHP Warning:  Undefined variable $new_lines in /usr/local/ispconfig/server/lib/classes/file.inc.php on line 126
PHP Stack trace:
PHP   1. {main}() /usr/local/ispconfig/server/server.php:0
PHP   2. modules->processDatalog() /usr/local/ispconfig/server/server.php:180
PHP   3. modules->raiseTableHook($table_name = 'shell_user', $action = 'u', $data = [...
PHP   4. web_module->process($tablename = 'shell_user', $action = 'u', $data = [...
PHP   5. plugins->raiseEvent($event_name = 'shell_user_update', $data = [...
PHP   6. shelluser_base_plugin->update($event_name = 'shell_user_update', $data = [...
PHP   7. shelluser_base_plugin->_setup_ssh_rsa() /usr/local/ispconfig/server/plugins-available/shelluser_base_plugin.inc.php:286
PHP   8. file->remove_blank_lines($input = '', $file = 0) /usr/local/ispconfig/server/plugins-available/shelluser_base_plugin.inc.php:463
DEBUG [shelluser base plugin.inc:529] - ssh-rsa key updated in /var/www/clients/client1/web27/home/testclientzfcbvz/.ssh/authorized_keys