Prevent potential null pointer error

lib/DataSet.js

@@ -241,6 +241,9 @@ DataSet.prototype.update = function (data, senderId) {
   var fieldId = me._fieldId;
 
   var addOrUpdate = function (item) {
+    if (!item){
+      return;
+    }
     var id = item[fieldId];
     if (me._data[id]) {
       var oldData = util.extend({}, me._data[id]);