Do not pass silently as we have a bug over here

@frapell this has to do with the implementation done in #451.

To replicate:

* add a carousel tile to a cover
* drop an item to it
* edit the tile to add a custom URL to the item
* edit the tile again and remove the custom URL
* drop another item into the tile

the first item is replaced by the second.

it would be great if you have time to check this and point us where the problem is; @rodfersou will work to fix it.