Cannot configure CKEditor 5 if site's themes are configured in a particular way

Woah, that sounds very bad!

If you can provide steps to reproduce, we can write tests that reproduce it and get this fixed 😊

As far as I can tell, this error is caused by failing to check for null values on line 79 of CKEditor5StylesheetsMessage.php.

The whole block reads as such:

    // Collect information on which themes/base themes have ckeditor_stylesheets
    // configuration, but do not have corresponding ckeditor5-stylesheets
    // configuration.
    $ckeditor_stylesheets_use = [];
    foreach ($themes as $theme_info) {
        $this->checkForStylesheetsEquivalent($theme_info, $ckeditor_stylesheets_use);
    }

The problem is that in this case $themes has an object in it that's null. I haven't looked into why this might be or what causes it because it doesn't seem to be causing errors elsewhere (yet). In my case, it was a 2-long array with one theme and one null.

Issue was fixed by checking for null values.

    // Collect information on which themes/base themes have ckeditor_stylesheets
    // configuration, but do not have corresponding ckeditor5-stylesheets
    // configuration.
    $ckeditor_stylesheets_use = [];
    foreach ($themes as $theme_info) {
        if (!empty($theme_info)) {
                $this->checkForStylesheetsEquivalent($theme_info, $ckeditor_stylesheets_use);
        }
    }

Thanks! That should allow a test to be written! 🤓

Any chance you're interested in doing so? :D

While this is being worked on, is there a way to fix this in my theme?

Installed Drupal 10.2.1 today. Claro as admin theme. Tried to set Ckeditor 5 for Full HTML. Could not. Had the message You have to configure Ckeditor 5. I noticed the message about fixing it using Claro. As an experiment, I used Olivero. That let me set Ckeditor 5 in Full HTML. I then switched back to Claro as the admin theme.

I then tried to create an article but Ckeditor would not appear for Full HTML using either Claro or Olivero.

I successfully added Ckeditor 5 to Basic HTML and the editor appeared in create Article.
I successfully added Ckeditor 5 to Restricted HTML and the editor appeared in create Article.

I successfully added Ckeditor 5 to Full HTML but the editor did not appear in create Article. I tested several add/delete/add cycles with Claro and Olivero but could not make Ckeditor work with Full HTML.

This might help with diagnostics and testing.

I also tested with a third party theme for the site but not admin. When I add Ckeditor 5 to Basic or Full or Restricted, there is a message about the theme having a style sheet for Ckeditor but not Ckeditor 5.

"The 0 Point theme has ckeditor_stylesheets configured without a corresponding ckeditor5-stylesheets configuration. See the change record for details."

Using Firefox console during create Article, I get the following message for Basic, restricted, and Full.
Uncaught TypeError: node is null
checkMutation http://example.com/node/add/article line 612 > eval:92

The line is:
node.nodeType === Node.ELEMENT_NODE &&
node.nodeName === 'SCRIPT' &&
node.dataset &&
node.dataset.bigPipeReplacementForPlaceholderWithId &&
typeof drupalSettings.bigPipePlaceholderIds[
node.dataset.bigPipeReplacementForPlaceholderWithId
] !== 'undefined',
);

When I switch to Full, I get this additional error:
CKEditorError: plugincollection-soft-required {"missingPlugin":"LinkEditing","requiredBy":"EntityEmbedLinkUi"}
Read more: https://ckeditor.com/docs/ckeditor5/latest/support/error-codes.html#erro...