How to present a Scene when touching the screen?

Question

My Game is in the GameScene and Menu in my MainMenu Scene.

I put the following code in the overridden touchesBegan of the MainMenu Scene to present the GameScene:

   let reveal = SKTransition.fade(withDuration: 0.5)
   let NextScene = GameScene(size: self.size)
   NextScene.scaleMode = SKSceneScaleMode.aspectFill
   self.view?.presentScene(NextScene, transition:reveal)

But if I test it I get the following error:

fatal error: unexpectedly found nil while unwrapping an Optional value

I just don't understand why because in the GameScene I have the follow code in the GameOver function which works perfectly fine:

    let reveal = SKTransition.fade(withDuration: 0.5)
    let MainMenuScene = MainMenu(size: self.size)
    MainMenuScene.scaleMode = SKSceneScaleMode.aspectFill
    self.view?.presentScene(MainMenuScene, transition:reveal)

They are the exact same but only the one in the GameOver Function works, why?