Nested Relationships not working properly when using popup form

Need Help
#1

I have a TestModels controller for a TestModel model and this config_relation.yaml:

items:
    label: Items
    view:
        list: $/utopigs/content/models/testmodelitem/columns.yaml
        toolbarButtons: create|delete
    manage:
        form: $/utopigs/content/models/testmodelitem/fields.yaml
items[subitems]:
    label: Subitems
    view:
        list: $/utopigs/content/models/testmodelsubitem/columns.yaml
        toolbarButtons: create|delete
    manage:
        form: $/utopigs/content/models/testmodelsubitem/fields.yaml

TestModel fields.yaml:

fields:
    title:
        label: Title
    items:
        label: Items
        type: relation

TestModelItem fields.yaml:

fields:
    title:
        label: Title
    subitems:
        label: Subitems
        type: relation

If I setup TestModel as normal form, it works perfectly. But if I configure it as a popup form, when I try to create a new subitem (or update an existing one) I get this error:

“Relation behavior used in Utopigs\Content\Controllers\TestModels does not have a model defined.”

Is this a bug, or I am missing something?