Update PreviewController

The constructor is called with the userId. However if a user is not
logged in this is null. Which means that we get an exception instead of
this being handled gracefully in the middleware.

There are cleaner solutions. But this is the solution that is the
easiest to apply without lots of work and risk of breaking things
(handling the logged in middleware before initializing the controller
etc).

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>