Fix crash when a track isn't loaded on ArtistAlbumWidget.get_songs

If you switch very fast from the 'Albuns' view to the 'Artists' view, it crashes. Apparently it happens when a track is not loaded yet, so methods such as get_url() return NoneType which cannot be converted to 'str' implicitly. It needs to be converted to 'str' explicitly to be used on logging.debug.