CLONE - debug_backtrace() sometimes do not return 'file' and 'line' values

debug_backtrace() sometimes do not return 'file' and 'line' values.
This results in an error (Notice).

Please, add "isset" in code: LoggerLoggingEvent.php, lines: 172-173

.....
$locationInfo['line'] = $hop['line'];
$locationInfo['file'] = $hop['file'];
.....

Thank you!