'Not Found' error on 3.1 means Widgets cannot be added

[MattyBalaam]

I have tried this on an old site and a fresh install.

I have tried using widgets both the old way, and using the new

Page:
extensions:
- WidgetPageExtension

method in my main config.yml file in mysite/_config

I can’t add widgets to a page using 3.1 and widgets. They have both been installed using Composer. When I click on the [+] button the CMS attempts to access url.com/WidgetController/EditableSegment/nameofWidget but gets a 404 error.

Any widgets already on the page work as expected.

If I use composer to roll back to 3.0.x everything works as expected again.

[MattyBalaam]

This issue seems to have disappeared with the newest versions of Silverstripe called in with "3.1.x-dev"