Update Core Data Value Swift

Hey I'm new to IOS programming and hers my problem: I have 2 tableView Controllers. The Main View Controller should display a value from a function from the second Table ViewController( In my case the average). In the prepare for segue I have this code:

 override func prepareForSegue(segue: UIStoryboardSegue,sender: AnyObject?) {

    if segue.identifier == "detail"{

        var selectedFach: NSManagedObject = daten[tableView.indexPathForSelectedRow()!.row] as NSManagedObject
        let IVC: NotenTVC = segue.destinationViewController as NotenTVC
        IVC.NotenSchnitt = selectedFach.valueForKey("Schnitt") as Double
        IVC.existirenderSchnitt = selectedFach

    }

}

And in my second ViewController I have this code :(Note that "averageOf(eingegebeneNote)" is my function)

var NotenSchnitt = 0.0
var existirenderSchnitt : NSManagedObject!
@IBOutlet weak var SchnittNotenLabel: UILabel!



 override func viewDidDisappear(animated: Bool) {
    //Update
    if averageOf(eingegebeneNote)! != 0{
    if existirenderSchnitt != nil{
      existirenderSchnitt.setValue(SchnittNotenLabel.text as String?, forKey: "Schnitt")
    }else{
    var neuesFach = NSEntityDescription.insertNewObjectForEntityForName("Fach", inManagedObjectContext: self.context!) as Fach
    neuesFach.schnitt = "\(self.averageOf(self.eingegebeneNote)!)"
        print(neuesFach.schnitt)
        print("saved")
        }
    }

My problem is that I get the error: fatal error: unexpectedly found nil while unwrapping an Optional value

In this line:

IVC.NotenSchnitt = selectedFach.valueForKey("Schnitt") as Double