@jnicholCU found the box plugin causing an error on Drupal 10.3 beta. I was able to reproduce the error on my Drupal 10.2 installation by compiling CKEditor 5 41.3.1 from source and linking the new @ckeditor/ckeditor5-essentials package. It's probably something we'll need to fix.
CKEditorError: selection.focus is null
Read more: https://ckeditor.com/docs/ckeditor5/latest/support/error-codes.html#error-selection.focus is null
getRelatedElement webpack://CKEditor5.box/./ckeditor5_plugins/box/src/boxui.js?:86
_updateToolbarsVisibility webpack://CKEditor5.dll/./packages/ckeditor5-widget/src/widgettoolbarrepository.ts?:122
...
@jnicholCU found the box plugin causing an error on Drupal 10.3 beta. I was able to reproduce the error on my Drupal 10.2 installation by compiling CKEditor 5 41.3.1 from source and linking the new
@ckeditor/ckeditor5-essentials
package. It's probably something we'll need to fix.