PHP Warning

Is there anyone who can address this PHP warning?

PHP Warning: The magic method jtmce\core\Singleton::__wakeup() must have public visibility in /home/customer/www/DOMAIN/public_html/wp-content/plugins/just-tinymce-styles/core/Singleton.php on line 60

Or are there any alternatives where we can add custom styles to a TinyMCE drop down in Classic Editor?