Reproduces problem with guide

app/controllers/ckeditors_controller.rb

@@ -0,0 +1,5 @@
+class CkeditorsController < ApplicationController
+  def show
+    render inline: "", :layout => "application"
+  end
+end

app/javascript/packs/Ckeditor.jsx

@@ -0,0 +1,33 @@
+import React, { Component } from 'react';
+import CKEditor from '@ckeditor/ckeditor5-react';
+import ClassicEditor from '@ckeditor/ckeditor5-build-classic';
+
+class App extends Component {
+    render() {
+        return (
+            <div className="App">
+                <h2>Using CKEditor 5 build in React</h2>
+                <CKEditor
+                    editor={ ClassicEditor }
+                    data="<p>Hello from CKEditor 5!</p>"
+                    onInit={ editor => {
+                        // You can store the "editor" and use when it is needed.
+                        console.log( 'Editor is ready to use!', editor );
+                    } }
+                    onChange={ ( event, editor ) => {
+                        const data = editor.getData();
+                        console.log( { event, editor, data } );
+                    } }
+                    onBlur={ ( event, editor ) => {
+                        console.log( 'Blur.', editor );
+                    } }
+                    onFocus={ ( event, editor ) => {
+                        console.log( 'Focus.', editor );
+                    } }
+                />
+            </div>
+        );
+    }
+}
+
+export default App;

app/javascript/packs/application.js → app/javascript/packs/application.jsx

@@ -15,3 +15,15 @@ require("channels")
 //
 // const images = require.context('../images', true)
 // const imagePath = (name) => images(name, true)
+
+import React from 'react'
+import ReactDOM from 'react-dom'
+import PropTypes from 'prop-types'
+import Ckeditor from './Ckeditor'
+
+document.addEventListener('DOMContentLoaded', () => {
+  ReactDOM.render(
+    <Ckeditor />,
+    document.body.appendChild(document.createElement('div')),
+  )
+})

config/routes.rb

@@ -1,3 +1,4 @@
 Rails.application.routes.draw do
+  resource 'ckeditor'
   # For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html
-end
+end