type () does not conform to protocol anyobject

I am trying to confirm submission before leaving the view controller :

however I am getting this error :

type () does not conform to protocol anyobject

here is the line where the error appears :

        self.navigationItem.backBarButtonItem?.target = self.validateBeforeBack("back", completion: { (bool) -> () in
        self.navigationController?.popViewControllerAnimated(true)
    })

via Chebli Mohamed