Magento 2.3 Upgrade : Undefined class constant 'WYSIWYG_EDITOR_CONFIG_PATH' in vendor/magento/module-ui/Block/Wysiwyg/ActiveEditor.php:56

Question

After Upgrading from Magento 2.2.7 to Magento 2.3, I am getting this error Undefined class constant 'WYSIWYG_EDITOR_CONFIG_PATH' in admin and frontend. Received this issue right after upgrading. The upgrade process was successful.

    <script>
    require.config({
        map: {
            '*': {
                wysiwygAdapter: '<br />
<b>Fatal error</b>:  Uncaught Error: Undefined class constant 'WYSIWYG_EDITOR_CONFIG_PATH' in /private/www/magento23.local/vendor/magento/module-ui/Block/Wysiwyg/ActiveEditor.php:56
Stack trace:
#0 /private/www/magento23.local/vendor/magento/module-ui/view/base/templates/wysiwyg/active_editor.phtml(18): Magento\Ui\Block\Wysiwyg\ActiveEditor-&gt;getWysiwygAdapterPath()
#1 /private/www/magento23.local/vendor/magento/framework/View/TemplateEngine/Php.php(59): include('/private/www/ic...')
#2 /private/www/magento23.local/vendor/magento/framework/View/Element/Template.php(271): Magento\Framework\View\TemplateEngine\Php-&gt;render(Object(Magento\Ui\Block\Wysiwyg\ActiveEditor), '/private/www/ic...', Array)
#3 /private/www/magento23.local/vendor/magento/framework/View/Element/Template.php(301): Magento\Framework\View\Element\Template-&gt;fetchView('/private/www/ic...')
#4 /private/www/magento23.local/vendor/magento/framework/View/Element/AbstractBlock.php(668): Magento\Framework\View\Element\Template-&gt;_toHtml()
#5 /private in <b>/private/www/magento23.local/vendor/magento/module-ui/Block/Wysiwyg/ActiveEditor.php</b> on line <b>56</b><br />

Answer 1

Check vendor/magento/module-ui/Model/Config.php file

In a fresh 2.3 installation that constant does exist

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Ui\Model;

use Magento\Framework\App\Config\ScopeConfigInterface;
use Magento\Framework\View\Element\Template;

class Config
{
    /**
     * Wysiwyg editor configuration path
     */
    const WYSIWYG_EDITOR_CONFIG_PATH = 'cms/wysiwyg/editor';

...

So, it seems that upgrade process didn't run as expected. Maybe you'd try it back