Inside a FieldConfigurator defined on an associated CollectionField's entity, $entityDto->getInstance() returns NULL

[nevez]

CRUD of entity "A" includes a CollectionField of entities "B". I'm trying to define a FieldConfigurator on B's fields. When I modify an entity of type "A" which has some associated "B"s, the configurator is correctly called also on B's fields: inside the "supports" method $entityDto->getFqcn() == B::class returns true, and $field->getProperty() returns their names, but $entityDto->getInstance() always returns null.

If instead I define the configurator on A, $entityDto->getInstance() correctly returns the instance, but I don't know how to access B's Fields and EntityDtos from there, to configure them.