In order for a CT to have a reference to an existing App, such as Assets, it is necessary to "retrofit" that app - that is, to create a CT for that existing app, and to decorate that existing app with a new class and a "contentType" field.

After that has been done, the YAML file for the new CT must be "touched". After this, the reference fields are displayed correctly in the CT-generated editor subapp.

But, after a restart of Magnolia, the reference fields are no longer displayed correctly when the editor is opened. The log displays:

ERROR agnolia.ui.form.field.factory.AbstractFieldFactory: Unknown configured type Asset

If the CT file is then "touched" again, the editor works as expected - the references work again.