Fetch Request: error . . . Core Data Swift 3 - ios

Question

Good Night;
Im creating an app that would save the name of people in a Core Data file, when I run the app and save what the user typed it works fine when I load (fetch ) the data it also works fine, the problem comes when I close or go to a different view controller when I hit the load data(fetch) the app crashed with : 'NSInvalidArgumentException',
reason: 'executeFetchRequest:error:
is not a valid NSFetchRequest.'
this is my code
#IBOutlet var PresidentTxt: UITextField!
#IBOutlet var SecDefenseTxt: UITextField!
let dataStack = CoreDataStack()
override func viewDidLoad() {
super.viewDidLoad()
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
#IBAction func Save(_ sender: Any) {
let database = dataStack.persistenContainer.viewContext
let dbrRecord = Command(context: database)
dbrRecord.potus = PresidentTxt.text
dbrRecord.defsec = SecDefenseTxt.text
do{
try database.save()
}catch {}
}
func didTapView(){
self.view.endEditing(true)
}
#IBAction func Load(_ sender: Any) {
let database = dataStack.persistenContainer.viewContext
do{
let results = try database.fetch(Command.fetchRequest()) as! [Command]
for presidentUS in results{
if let _ = presidentUS.potus {
PresidentTxt.text = presidentUS.potus
}
}
for DefenseSec in results {
if let _ = DefenseSec.defsec {
SecDefenseTxt.text = DefenseSec.defsec
}
}
}catch {}
}