Problem/Motivation
We were trying to embed content into ckedtor. Didn't work, the drupal-entity tag was not added to editor content.
When core/modules/editor/src/Ajax/EditorDialogSave.php renders, the attribute 'selector' is missing in the data returned to the client.
In drupal/html/core/modules/ckeditor5/js/ckeditor5.js, the 'editor:dialogsave' handler responsible for adding the embedded content requires attribute 'selector' to be set.
Proposed resolution
The attached patch solved our problem.