First check if callback is defined before calling it

Author: jochen-testingbot

lib/api.js

@@ -162,7 +162,9 @@ TestingBot.prototype.request = function(req_data, callback) {
     } else {
       error = response.body;
     }
-    callback(error, responseBody);
+    if (callback) {
+      callback(error, responseBody);
+    }
   });
 };