This was triggered by having {{ tinymce_init() }} in a javascript block.

An exception has been thrown during the rendering of a template ("Catchable Fatal Error: Argument 1 passed to Symfony\Bundle\FrameworkBundle\Templating\Helper\AssetsHelper::__construct() must be an instance of Symfony\Component\Asset\Packages, string given, called in /my/directory/app/cache/dev/appDevDebugProjectContainer.php on line 5370 and defined"