As explained in sensiolabs/SensioDistributionBundle#265:
Deprecation Notice: The callback Stfalcon\Bundle\TinymceBundle\Composer\ScriptHandler::createSymlink declared at vendor/gibilogic/tinymce-bundle/src/Composer/ScriptHandler.php accepts a Composer\Script\CommandEvent but post-update-cmd events use a Composer\Script\Event instance. Please adjust your type hint accordingly, see https://getcomposer.org/doc/articles/scripts.md#event-classes
As explained in sensiolabs/SensioDistributionBundle#265: