check for scope.opts.timefield, return early if missing

Fixes #5509

src/plugins/kibana/public/discover/controllers/discover.js

@@ -191,6 +191,9 @@ define(function (require) {
         });
 
         $scope.$watch('vis.aggs', function () {
+          // no timefield, no vis, nothing to update
+          if (!$scope.opts.timefield) return;
+
           var buckets = $scope.vis.aggs.bySchemaGroup.buckets;
 
           if (buckets && buckets.length === 1) {