bitrise-workflow-editor: Error loading steplib specs / Missing output data

When I try to run workflow-editor 0.9.1 on my bitrise step repos (ex. aws-device-farm-test-packager) I get the following error:

Js console shows:

Error: Error loading steplib specs.
    at main.js:53458
    at processQueue (main.js:27448)
    at main.js:27464
    at Scope.$eval (main.js:28716)
    at Scope.$digest (main.js:28532)
    at Scope.$apply (main.js:28824)
    at done (main.js:23162)
    at completeRequest (main.js:23360)
    at XMLHttpRequest.requestLoaded (main.js:23301)

Terminal shows:

$ bitrise :workflow-editor
Starting API server at http://localhost:3645
Open workflow editor in browser ...
 => GET: /api/bitrise-yml.json - 14.842118ms ()
Missing output data
 => GET: /api/spec - 40.626267ms ()