The Diagram Viewer & Diagram Editor widgets are not working, once trying to open some diagram instance an error popup appears.

NOTE: Set minor priority due to Diagram Editor/Viewer components are not being used on the workbench editors for processes. The wb editors are using Session Editor/Viewers, which are actually not causing issues

First issue analysis
See method getPreferences(diagram) in AbstractDiagramViewer class - it relies on having the preferences set in the preferences registry, otherwise it returns null.
On the other hand, the preferences are actually only set in the preferences registry by the SessionLoader, which is not a valid actor when using the Diagram Viewer/Editor widget, as no session is currently active / present for it.