Very similar to this issue [#3094397], it appears another hash is missing in the same file (seven.theme), a few lines down. When attempting to add a remote video to a field, User error: "0" is an invalid render array key in Drupal\Core\Render\Element::children() was showing up the dblog several times. Adding the hash here $form['#attributes']['class'][] = 'media-library-add-form--oembed'; stopped the error from appearing. Steps to reproduce: