Error in widget.js/dojo.js in custom navigation pane after upgrade (to CMS UI 8.5.1)

Description

A custom navigation pane for the Commerce site has been developed. It was working in a previous version (8.9). They upgraded to 8.15 and later but their custom widget/navigation pane doesn't work.

I implemented their code samples to a vanilla 8.16.1 site and was able to reproduce the same dojo errors they were getting in console. I don't see anything wrong in their code (technically speaking) - perhaps there was some breaking change in the UI between Commerce 8.9 and 8.15 and later

Error message (or get something similar) in Developer Tools:

The store was queried with a [string] but the entity id is a [undefined]". The [string] variable has the value "-1_-1".

"Observable store detected, but no sort order specified. You may experience quirks when adding/updating items. These can be resolved by setting a sort order on the list or grid."

Attached are their .cs files. Add and implement code to a vanilla 8.15 or later Commerce sample site (webforms) and you'll reproduce the errors.

Related links:

https://www.epinova.no/blog/arve-systad/dates/2014/9/custom-icontent-with-its-own-navigation-pane/
http://world.episerver.com/forum/developer-forum/-EPiServer-75-CMS/Thread-Container/2014/9/Custom-IContent-type-with-asset-pane-wont-show-For-this-X/

Note

There was a change in the internal ShardBlocks gadget in EPiServer.CMS.UI 8.2.1 that required the property EnableContextualContent to be set to true on the ContentRepositoryDescriptor to be able to select the local assets folder.