Getting error on loading atlassian-connect add-on's configuration page

I am trying to develop a Jira add-on for my SAAS product.
I am stuck trying to load the configuration page.
I am getting an error which says:

 Something's gone wrong
 Our team has been notified. If the problem persists, please contact Atlassian Support.

image

I have tried using this sample application - Bitbucket