fyi, the plugin is currently not compatible with the latest tests-passed version 3.1.0.beta2( d33d98f9c1 ) of Discourse.
I had to disable it in app.yml in our Discourse instance otherwise the rebuild of Discourse did fail.
fix has been implemented by @merefield
https://github.com/paviliondev/discourse-custom-wizard/pull/220
Thanks you guys for the quick fix.
This topic can now be closed.