TypeError: array_intersect(): Argument #1 ($array) must be of type array, string given in array_intersect() (line 203 of .../core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php).

The steps to reproduce describe a normal activity so there must be more setup needed to reproduce the bug.

Also, please provide the backtrace.

Also, which contrib modules did you install that provide CKEditor 5 plugins? It's likely that is the root cause.

In case this helps anyone, we were having a similar error:

TypeError: array_intersect(): Argument #1 ($array) must be of type array, null given in array_intersect() (line 203 of        
                                           web/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php

In our case, it happened because we updated our site to Drupal 10 but the configuration remained unchanged from our Drupal 9 previous version of the site. Then, the config element of the editor had a different structure that the CKEditor5PluginManager could not iterate.

We solved it by either editing and saving the text format again at /admin/config/content/formats, or deleting the faulty text format and creating it again (where the previous solution was not possible).

@anacolautti Could you please share the exported text format + editor config entities from both before and after the re-saving? 🙏

Hi @wim-leers. Sure, I hope this helps!

I believe this structure change

d9 settings:

settings
  toolbar
    rows 

to

d10 settings:

settings
  toolbar
    items 

is what broke my site. The error line was this one:

web/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php

if (empty(array_intersect($editor->getSettings()['toolbar']['items'], array_keys($definition->getToolbarItems())))) {
  unset($definitions[$plugin_id]);
}

and the error, that the first argument of array_intersect could not be null

Thanks, @anacolautti! Clarifying that this is no longer blocked.

Decreasing priority though because it seems very few people are running into this 😇

Actually, … investigated right away, because I was too curious to find out how this could fail 🤓

Yes, you're right, d9.editor.editor.basic_html.yml is plain wrong: its settings are for CKEditor 4, not 5!

Drupal cannot have done this itself. In #5 you wrote:

We solved it by either editing and saving the text format again at /admin/config/content/formats, or deleting the faulty text format and creating it again (where the previous solution was not possible).

… that seems to have been the culprit.

In our case, it happened because we updated our site to Drupal 10 but the configuration remained unchanged from our Drupal 9 previous version of the site. Then, the config element of the editor had a different structure that the CKEditor5PluginManager could not iterate.

How did you go from Drupal 9 to 10? From which exact version to which exact version? Was https://www.drupal.org/project/ckeditor installed?

There are very explicit instructions during the update from 9 to 10. Although #3304736-43: Provide a good UX to ensure the CKEditor 4 to 5 update is always done before upgrading to Drupal 10 unless the contrib module is installed unfortunately never landed, and that would've made it more clear still 🙁 I definitely tried to convince core committers! 😅

So … I'm afraid I once again need more detail to actually be able to harden against this 🫣

I think this would have been prevented by #2628144: Ensure that ConfigImport is taking place against the same code base

Thanks, @catch, TIL.

Per #8 and #10.

Regarding this topic, just a small update.

I had a similar error and the root cause in our project was the config for "config/sync/editor.editor.webform_default.yml".

In our case, this configuration was not updated when we updated to ckeditor5, and after migrating to Drupal 10, we were having the error in this code:

web/core/modules/ckeditor5/src/Plugin/CKEditor5PluginManager.php

 // Otherwise, only remove the definition if the plugin has buttons and
      // none of its buttons are active.
      elseif ($definition->hasToolbarItems()) {
        if (empty(array_intersect($editor->getSettings()['toolbar']['items'], array_keys($definition->getToolbarItems())))) {
          unset($definitions[$plugin_id]);
        }
      }
    }

Just in case, I've found a workaround.
Adding these lines into settings section of editor.editor.*.yml files fixed the issue for me:

plugins: {  }
toolbar:
  items: {  }

Many thanks to @anacolautti.