In bpmn-js every shape and connection has a businessObject property which is the actual model element with all the business logic. The error occurs because bpmn-js is trying to create a business object for your custom element but it doesn’t know how this is supposed to look like. In the custom elements example there is a custom element factory which makes sure that for custom elements like the triangle and the circle no business objects are created. If you’d want to do that you’d have to add the definitions of how this is supposed to look like. We have an example for this as well.