InstallTrigger.getVersion return null?

[MacTan]

How does InstallTrigger.getVersion("??") work???

I created a web page with 2 button, one for trigger the download and another
to get the version number.
If I trigger the download and then call getVersion(), it will return
"1.0.1.0".
However, if I restart the browser and call getVersion directly without
triggering the download, it always return null even the package is already
installed. Why??

any help. thanks

err = initInstall("barley v", "barley", "1.0.1.0");
function installxpi()
{

InstallTrigger.startSoftwareUpdate("http://localhost/project/mozilla/xpInsta
ll/barley.xpi",0);
}

function getversion1()
{
 mrscTrigger = InstallTrigger.getVersion('barley');
 alert(mrscTrigger);
}