I assume that this is because of a mixture of ES modules and the old CommonJS modules. The properties panel module doesn’t seem to be an ES module. So in the code I’m importing these modules using:
export const _EntryFactory = require('bpmn-js-properties-panel/lib/factory/EntryFactory');
instead of
import _EntryFactory from 'bpmn-js-properties-panel/lib/factory/EntryFactory';
I got it running using these changes. I’ll try to upload my sample this evening.