title: CKeditor 5 source editing plugin
body: <p>I'm using CKEditor 5, and I want to install an additional plugin, 'source editing', but as far as I try, it doesn't work out for me. If I use CKEditor without
  additional plugin, then everything works as it should, but when I try to register the source editing plugin, I get the following error:</p>
  <p><a href="https://i.stack.imgur.com/WJlA3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WJlA3.png" alt="enter image description here" /></a></p>
  <p>My code:</p>
  <pre><code>const ClassicEditor = window.ClassicEditor = require('@ckeditor/ckeditor5-build-classic');
  const SourceEditing = window.SourceEditing = require('@ckeditor/ckeditor5-source-editing');
  ClassicEditor
          .create(document.querySelector('#editor'), {
              plugins: [SourceEditing], // &lt;--- Trying to add a plugin to a build
              toolbar: ['sourceEditing'] // &lt;--- Trying to add a plugin to a build
          })
          .then(editor =&gt; {
              console.log(editor);
          })
          .catch(error =&gt; {
              console.error(error);
          });
  
  </code></pre>
  <p>The versions of SourceEditing and CKEditor are the same.</p>
  <p><a href="https://i.stack.imgur.com/Jbcus.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Jbcus.png" alt="enter image description here" /></a></p>