supplimental fix for #102180. when closing the stand alone message window, we were

not unregistering the folder listener from the mail session, because the unload handler
was failing.  it was failing because GetSearchSession() has a js error, because
for the stand alone msg window, gSearchSession is undefined.  r/sr=bienvenu

mailnews/base/resources/content/mailWindow.js

@@ -559,7 +559,7 @@ function OpenInboxForServer(server)
 
 function GetSearchSession()
 {
-  if (gSearchSession)
+  if (("gSearchSession" in top) && gSearchSession)
     return gSearchSession;
   else
     return null;