How do a control alert for a TextField in Swift 3

I have two UITextFields and want to save the Input in Core Data. If the TextField is empty there should be an Alert like "Please insert value".

I try to do this:

@IBAction func buttonSave(_ sender: AnyObject) {

    let db:DB = NSEntityDescription.insertNewObject(forEntityName: "DB", into: DatabaseController.persistentContainer.viewContext) as! DB

    if timeFextField.text == "" {


        let dateAlert = UIAlertController(title: "Missung Value", message: "Please insert a date.", preferredStyle: UIAlertControllerStyle.alert)

        let okAction = UIAlertAction(title: "OK", style: .default){(action: UIAlertAction) in print("OK")
        }

        dateAlert.addAction(okAction)

        self.present(dateAlert, animated: true, completion: nil)

    } else {
        if labelTypNumber.text == "" {
            let typeAlert = UIAlertController(title: "Missung Value", message: "Please select a type.", preferredStyle: UIAlertControllerStyle.alert)

            let okAction = UIAlertAction(title: "OK", style: .default){(action: UIAlertAction) in print("OK")
            }

            typeAlert.addAction(okAction)

            self.present(typeAlert, animated: true, completion: nil)
        }
        else {

            db.time = NSDate()
            db.date = timeFextField.text!
            db.typ = Double(labelTypNumber.text!)!
            db.color = colorTextField.text!
            db.notes = notesTextField.text!
            db.weather = wetterTextField.text!

            if mediSegment.selectedSegmentIndex == 0 {
                db.medi = mediTextField.text!
            } else {
                db.medi = "0"
            }

            if mediSegment.selectedSegmentIndex == 1 {
                db.medi2 = mediTextField.text!
            } else {
                db.medi2 = "0"
            }

            DatabaseController.saveContext()

            navigationController!.popViewController(animated: true)

        }
    }


}

Now when the UITextField is empty, I get an UIAlert, but when I load the Data from Cora Data, the App crashes.

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

It looks like, that the values are stored in Core Data even though the alert is showing. Have anyone an idea to fix this problem?

Thanks, many greetings.