'Getting started' Button extension doesn't work

Hi

I’m new to BitBucket development and am trying to get the basic ‘getting started’ samples to work - specifically the Button sample …

https://developer.atlassian.com/server/framework/clientside-extensions/reference/api/extension-factory/#button

The project builds just fine. However when I try and navigate to the pull-request overview page - there’s a ‘This page couldn’t be displayed’ graphic in the page. Reviewing the Javascript console - I can see the following error message …

“Warning: Failed prop type: The prop onRender is marked as required in ExtensionModal, but its value is undefined.”

What’s wrong and how do I fix it ?

Thanks
Dave