WatchKit present a SKScene from another unexpectedly found nil while unwrapping an optional value

Question

I want to present a SpriteKit scene from another SKScene in my WatchKit app. Here is my code:

class GameScene: SKScene, SKPhysicsContactDelegate {
var scene2: Scene2!
@IBOutlet var skInterface: WKInterfaceSKScene!
override func sceneDidLoad() {
    physicsWorld.contactDelegate = self
 ....
 changeScene()

 }
func changeScene() {
if let scene = Scene2(fileNamed: "Scene2") {

        scene2 = scene


        scene.scaleMode = .aspectFill


        self.skInterface.presentScene(scene)


        self.skInterface.preferredFramesPerSecond = 30
    }

} But when i try to present other scene i get this error:

   "fatal error: unexpectedly found nil while unwrapping an Optional value" 

How can i solve this problem?