The application may fail to load. If so, a TypeError is returned in Firebug.
This is a known issue.
Note: As of April 2018, the Maps and Apps Gallery app template is in the Mature Phase of the ArcGIS Online Product Lifecycle. For more information regarding support of this app template, refer to the Maps and Apps Gallery requirements page. It is recommended to use the Minimal Gallery app template instead of Maps and Apps Gallery.
Press F5 to refresh the browser and reload the application. Successfully loading the application once lessens the chances of encountering the error again.