Fix crash with TankPublishedFile without thumbnails

In some case, a TankPublishedFile can not have a thumbnail uploaded. Either the entity was manually created or the studio have modified the core to prevent "placeholder" thumbnail from being uploaded, saving screen space in the web-interface. In that case, a thread in the BrowserWidget will crash.
Merge pull request #3 from renaudll/patch-1

Fix crash with TankPublishedFile without thumbnails