Wert des Typs 'String' hat kein Mitglied 'stringByTrimmingCharactersInSet'

Nachdem ich mein Projekt nach Swift 3 konvertiert habe, erhalte ich in der ersten Zeile dieses Blocks den folgenden Value of type 'String' has no member 'stringByTrimmingCharactersInSet'-Fehler:

extension UIColor {
    convenience init (hex:String) {
        var cString:String = hex.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet()).uppercased() // error appears on this line

        if (cString.hasPrefix("#")) {
            cString = (cString as NSString).substring(from: 1)
        }



        let rString = (cString as NSString).substring(to: 2)
        let gString = ((cString as NSString).substring(from: 2) as NSString).substring(to: 2)
        let bString = ((cString as NSString).substring(from: 4) as NSString).substring(to: 2)

        var r:CUnsignedInt = 0, g:CUnsignedInt = 0, b:CUnsignedInt = 0;
        Scanner(string: rString).scanHexInt32(&r)
        Scanner(string: gString).scanHexInt32(&g)
        Scanner(string: bString).scanHexInt32(&b)


        self.init(red: CGFloat(r) / 255.0, green: CGFloat(g) / 255.0, blue: CGFloat(b) / 255.0, alpha: CGFloat(1))
    }
} 

Ich vermute, der Fehler rührt von einer Syntaxänderung mit 'stringByTrimmingCharactersInSet' her. Was ist die Korrektur dafür?