Adding persistent subscription support

src/common/utils/ensure.js

@@ -4,3 +4,8 @@ module.exports.notNullOrEmpty = function(value, name) {
   if (value === '')
     throw new Error(name + " is empty.");
 };
+
+module.exports.notNull = function(value, name) {
+  if (value === null)
+    throw new Error(name + " is null.");
+};