Undefined error trying to save settings

[dhandler]

When I try and make changes to the settings through Admin Tool -> Settings, after hitting SAVE, I get the following error:

( ! ) Notice: Undefined variable: xml in C:\wamp\www\LetoDMS\inc\inc.ClassSettings.php on line 424
Call Stack
# Time Memory Function Location
1 0.0007 754528 {main}( ) ..\op.Settings.php:0
2 0.0482 7596504 Settings->save( ) ..\op.Settings.php:133
3 0.0493 7597616 Settings->getXMLNode( ) ..\inc.ClassSettings.php:554

Line 554 from the file reads as:

$node = $this->getXMLNode($xml, '/configuration/system', 'smtp');

Any ideas?