Fail gracefully when singular selected item disappears

(This was causing a crash in the walkthrough if the user failed to connect
Flower Street and the walkthrough removed the new way and restarted the task)

js/id/modes/select.js

@@ -27,7 +27,7 @@ iD.modes.Select = function(context, selectedIDs) {
 
     function singular() {
         if (selectedIDs.length === 1) {
-            return context.entity(selectedIDs[0]);
+            return context.hasEntity(selectedIDs[0]);
         }
     }