Problem/Motivation
1) Go to admin/config/content/formats/add.
2) Add a test editor 'test_format' using CKEditor as the text editor.
3) Press 'Save Configuration'.
4) Go to /admin/config/content/formats/manage/test_format
5) Deselect CKEditor from the "Text editor" select dropdown
6) Select CKEditor from the "Text editor" select dropdown
Expected behavior: CKEditor subform appears
Actual: Ajax error
TypeError: Argument 1 passed to Drupal\Core\Form\SubformState::createForSubform() must be of the type array, null given, called in /Users/oknate/dev/d8test/web/core/modules/editor/editor.module on line 222 in Drupal\Core\Form\SubformState::createForSubform() (line 56 of /Users/oknate/dev/d8test/web/core/lib/Drupal/Core/Form/SubformState.php).
Proposed resolution
1) Verify subform exists before trying to validate it
2) There's actually a second bug here, which is it validates the CKEditor subform when switching to "None". So it should also check that the editor is not empty.
