Error after updating Webform from 8.x-5.0-beta15 to 8.x-5.0-rc1
Yes, flushing the cache via Drush should solve the problem. 

If you can't use Drush, you must put your site in maintenance mode, set $update_free_access=TRUE, and run the pending updates via /update.php. 

Also see 

http://www.zymphonies.com/blog/update-drupal8-core
https://www.drupal.org/project/webform/issues/2907960