Closing extended TrueCrypt volumes

When the nautilus script to close an extended TrueCrypt volume is executed on a container file, the optional automated backup will not work.
This is because the script used the variable NAUTILUS_SCRIPT_SELECTED_FILE_PATHS, assuming this is the mountpoint of the extended TrueCrypt volume. This is only true if the script is executed on the mounted container icon on the desktop. Closing the container would work in either case, since TrueCrypt successfully unmounts the volume no matter if you pass a mountpoint or a file path, but the backup part of our script did not work.