MediaModal callback sometimes returns an array

[gilbarbara]

Hello,

Thanks for this repo, it's a life saver!
I'm having some errors in this new site that I'm working. If I remove the selected image and try to add it again I'm getting a WordPress alert with "Could not set that as the thumbnail image.."

I was trying to see what was going on and the MediaModal callback attachment parameter is an array instead of an object for the subsequent calls.

cb : function(attachment){
    MultiPostThumbnails.setAsThumbnail(attachment.id, "%2$s", "%1$s", "%4$s");
}

I was able to fix with attachment.id || attachment[0].id for setAsThumbnail