Return 'null' explicitly

lib/cartodb/models/mapconfig/provider/create-layergroup-provider.js

@@ -44,7 +44,7 @@ CreateLayergroupMapConfigProvider.prototype.getMapConfig = function(callback) {
         context.limits = renderLimits;
         this.context = context;
 
-        return callback(err, this.mapConfig, this.params, context);
+        return callback(null, this.mapConfig, this.params, context);
     });
 };
 

lib/cartodb/models/mapconfig/provider/map-store-provider.js

@@ -28,33 +28,30 @@ function MapStoreMapConfigProvider(mapStore, user, userLimitsBackend, pgConnecti
 module.exports = MapStoreMapConfigProvider;
 
 MapStoreMapConfigProvider.prototype.getMapConfig = function(callback) {
-    var self = this;
-
     if (this.mapConfig !== null) {
         return callback(null, this.mapConfig, this.params, this.context);
     }
 
     var context = {};
 
-    step(
-        function prepareContextLimits() {
-            self.userLimitsBackend.getRenderLimits(self.user, self.params.api_key, this);
-        },
-        function handleRenderLimits(err, renderLimits) {
-            assert.ifError(err);
-            context.limits = renderLimits;
-            return null;
-        },
-        function loadMapConfig(err) {
-            assert.ifError(err);
-            self.mapStore.load(self.token, this);
-        },
-        function finish(err, mapConfig) {
-            self.mapConfig = mapConfig;
-            self.context = context;
-            return callback(err, mapConfig, self.params, context);
+    this.userLimitsBackend.getRenderLimits(this.user, this.params.api_key, (err, renderLimits) => {
+        if (err) {
+            return callback(err);
         }
-    );
+
+        context.limits = renderLimits;
+
+        this.mapStore.load(this.token, (err, mapConfig) => {
+            if (err) {
+                return callback(err);
+            }
+
+            this.mapConfig = mapConfig;
+            this.context = context;
+
+            return callback(null, mapConfig, this.params, context);
+        });
+    });
 };
 
 MapStoreMapConfigProvider.prototype.getKey = function() {