Interrupting SceneKit scene on app entering background

When my game enters background mode, I would like to pause completely the game. At the moment this is what I do:

In the AppDelegate:

func applicationWillResignActive(application: UIApplication) {
    gameViewControllerDelegate?.pauseGame()
}

In my game controller:

func pauseGame() {
    buttonPausePressed(buttonPausePlay)
}

func buttonPausePressed(sender: UIButton!) {
    scnView?.scene?.paused = true
    stopMusic()
    let exampleImage = UIImage(named: "16-play")?.imageWithRenderingMode(.AlwaysTemplate)
    sender.setImage(exampleImage, forState: UIControlState.Normal)
}

The method gets called and the image of the button is changed. Even the game is paused. But when I open the app again and un-pause it using:

scnView?.scene?.paused = false

all the graphics changes and other weird things happen. It seems like the SCNActions have never been paused. Any ideas?