Update 4.1.6 to 4.2.3 breaks site

Hi there,

We're seeing an issue with shortcode core that breaks the live site - and I'm not sure how to begin diagnosing!

We're running Grav 1.6.25 - and this morning updated all plugins to the latest version. That update migrated shortcode code from 4.1.6 to 4.2.3 - but after the update we see this message on the live site:


With this message in the logs:
```
[2020-05-27 09:52:41] grav.CRITICAL: Class 'Thunder\Shortcode\EventContainer\EventContainer' not found - Trace: #0 D:\home\site\wwwroot\user\plugins\shortcode-core\shortcode-core.php(64): Grav\Plugin\ShortcodeCore\ShortcodeManager->__construct() #1 D:\home\site\wwwroot\vendor\symfony\event-dispatcher\EventDispatcher.php(212): Grav\Plugin\ShortcodeCorePlugin->onPluginsInitialized(Object(RocketTheme\Toolbox\Event\Event), 'onPluginsInitia...', Object(RocketTheme\Toolbox\Event\EventDispatcher)) #2 D:\home\site\wwwroot\vendor\symfony\event-dispatcher\EventDispatcher.php(44): Symfony\Component\EventDispatcher\EventDispatcher->doDispatch(Array, 'onPluginsInitia...', Object(RocketTheme\Toolbox\Event\Event)) #3 D:\home\site\wwwroot\vendor\rockettheme\toolbox\Event\src\EventDispatcher.php(23): Symfony\Component\EventDispatcher\EventDispatcher->dispatch('onPluginsInitia...', Object(RocketTheme\Toolbox\Event\Event)) #4 D:\home\site\wwwroot\system\src\Grav\Common\Grav.php(403): RocketTheme\Toolbox\Event\EventDispatcher->dispatch('onPluginsInitia...', Object(RocketTheme\Toolbox\Event\Event)) #5 D:\home\site\wwwroot\system\src\Grav\Common\Processors\PluginsProcessor.php(27): Grav\Common\Grav->fireEvent('onPluginsInitia...') #6 D:\home\site\wwwroot\system\src\Grav\Framework\RequestHandler\Traits\RequestHandlerTrait.php(45): Grav\Common\Processors\PluginsProcessor->process(Object(Nyholm\Psr7\ServerRequest), Object(Grav\Framework\RequestHandler\RequestHandler))
```

For those people seeing the error if they clear their local cookies the issue goes away! I've able to repeat that with several people - each time clearing different degrees of local storage.

For now I've had to revert the site back to the older version - but I'm puzzled why this issue happens. Any idea?

Thanks!