Fix crash when selecting a category preset with enter button

(this happens because the datum() is a constructor not a constructed object, and
category presets have state, so calling datum().choose from keypress doesn't work)

js/id/ui/preset_list.js

@@ -154,6 +154,8 @@ iD.ui.PresetList = function(context) {
         }
 
         item.choose = function() {
+            if (!box || !sublist) return;
+
             if (shown) {
                 shown = false;
                 box.transition()