fix for bug #145580, JS errors in mailWindowOverlay.js for newgroups only configuration

r=bz, sr=sspitzer, a=sspitzer, thanks to ian@arlen.demon.co.uk for the patch.

mailnews/base/resources/content/mailWindowOverlay.js

@@ -1562,14 +1562,14 @@ function IsEmptyTrashEnabled()
 {
   var folderURI = GetSelectedFolderURI();
   var server = GetServer(folderURI);
-  return (server.canEmptyTrashOnExit?IsMailFolderSelected():false);
+  return (server && server.canEmptyTrashOnExit ? IsMailFolderSelected() : false);
 }
 
 function IsCompactFolderEnabled()
 {
   var folderURI = GetSelectedFolderURI();
   var server = GetServer(folderURI);
-  if (!(server.canCompactFoldersOnServer))
+  if (!(server && server.canCompactFoldersOnServer))
     return false;
 
   var folderTree = GetFolderTree();