Move null check in tool code one level out.  This means it will guard against trying to use worldedit for querying double chests when worldedit is not loaded. Fixes #511

Long-term, perhaps rather than using a worldedit object directly, we should use our own interface which supports similar methods and have two implementations, one which uses a worldedit selection and one which takes two corner coordinates for use in double chests.  That might also allow for custom queries along those lines.