Problem with javascript-code after unload thunderbird

[Steff81]

hi
i'm using the function

<script>
window.addEventListener("unload", autoExportOnClose, false);
</script>

in my xul overlay for thunderbird to start my automatic export from
lightning.
but it doesn't work with failure...

Fehler: [Exception... "'[JavaScript Error: "autoExportPrefs is not
defined" {file: "chrome://autoexport/content/autoExport.js" line:
116}]' when calling method:
[calIOperationListener::onOperationComplete]" nsresult: "0x80570021
(NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS)" location: "JS frame ::
file:///D:/11%20AppData/Thunderbird/Steffen/extensions/%7Be2fda1a4-762b-4020-b5ad-a41df1933103%7D/modules/calProviderUtils.jsm
:: cPB_notifyOperationComplete :: line 600" data: yes]

line 116 ...
> if (autoExportPrefs.getBoolPref(mode + ".exportFormat.ics"))
this failure occurs although there is the global definition...
> var autoExportPrefs = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefService).getBranch("ae.prefid.");

this failure doesn't occur if i start the export with a button during
thunderbird is running with the same code.
and this is new with thunderbird 3

has anyone an idea?

thanks in advance
regards
steff