Fixed thumbnail cache not being rebuilt anymore in "Show video preview in thumbnails" mode

Hi, I just noticed that the thumbnails were not generated anymore in the Show Video Preview in Thumbnails mode. I kind of like this feature. So I tracked it down to 61cba5e2. It appeared to be an erroneous inversion so that CacheTask::run got aborted when the task manager was not blocked. Removing the inversion fixed this. Ok.

Have a nice day, Ondrej