fatal error: unexpectedly found nil while unwrapping an Optional value

//
//  PlayList.swift
//  Algoryrhythm
//
//  Created by david on 3/09/15.
//  Copyright (c) 2015 Tomcorp. All rights reserved.
//

import Foundation
import UIKit
//data model

// llamar cada uno de los items
struct PlayList {
    var title: String?
    var description: String?
    var icon: UIImage?
    var largeIcon: UIImage?
    var artist: [String] = []
    var backgroundColor: UIColor = UIColor.clearColor()

    //inicializador de musiclibrary
    init(index: Int) {
        let musicLibrary = MusicLibrary().library
        let playlistDictionary = musicLibrary[index]
       // llamr cada item del otro view
        title = playlistDictionary["title"] as! String!
        description = playlistDictionary["description"] as! String!
        let iconName = playlistDictionary["icon"] as! String!
        icon = UIImage(named: iconName)
        let largeIconName = playlistDictionary["largeIcon"] as! String!
        largeIcon = UIImage(named: largeIconName)
        artist += playlistDictionary["artist"] as! [String]  //HERE ITS THE PROBLEM someone can help???? i have to put each as with ! symbol because xcode tell me to do it 
        let colorsDictionary = playlistDictionary["backgroundColor"] as! [String: CGFloat]
        backgroundColor = rgbColorFromDictionary(colorsDictionary)
    }


    func rgbColorFromDictionary(colorDictoniary: [String: CGFloat]) -> UIColor {
        let red = colorDictoniary["red"]!
        let green = colorDictoniary["green"]!
        let blue = colorDictoniary["blue"]!
        let alpha = colorDictoniary["alpha"]!

        return UIColor(red: red/255.0, green: green/255.0, blue: blue/255.0, alpha: alpha)


    }

}