How to nest a field collection?
Using the Field collection module in Drupal 8 seems to NOT be a good idea anymore. It currently only has an alfa1 release from April 2016). Instead you may want to try to use Paragraphs instead, as suggested also on the Field collection project page, i.e.:


  Paragraphs is likely to replace field collection for Drupal 8. Field collection is on its way to being deprecated. It is recommended to use paragraphs instead of field collection for Drupal 8 projects.