Add placeholder markup in preview mode

For components that do not have any rendered output when no content item is selected (eg. the video component) there should still be something visible in the channel manager.

You can use the isPreview() method to determine that you are in the channel manager and add some markup saying "No content item selected, please select a content item"

This way the webmaster knows where the component he just dropped is, instead of that awkward empty yellow box.