web: show a proper not found page on 404

hledger-web/Foundation.hs

@@ -235,17 +235,19 @@ viewdataWithDateAndParams d q a p =
 -- | Gather data used by handlers and templates in the current request.
 getViewData :: Handler ViewData
 getViewData = do
+  mhere      <- getCurrentRoute
+  case mhere of
+    Nothing -> return nullviewdata
+    Just here -> do
       app        <- getYesod
       let opts@WebOpts{cliopts_=copts@CliOpts{reportopts_=ropts}} = appOpts app
       today      <- liftIO getCurrentDay
       (j, merr)  <- getCurrentJournal app copts{reportopts_=ropts{no_elide_=True}} today
       lastmsg    <- getLastMessage
       let msg = maybe lastmsg (Just . toHtml) merr
-  Just here  <- getCurrentRoute
       q          <- getParameterOrNull "q"
       a          <- getParameterOrNull "a"
       p          <- getParameterOrNull "p"
-
       -- sidebar visibility: show it, unless there is a showsidebar cookie
       -- set to "0", or a ?sidebar=0 query parameter.
       msidebarparam <- lookupGetParam "sidebar"