SwiftUI + CloudKit Try to add a record with a pop up text field

I am trying to add a record which contains the order and text of it. I can add a new record within the same file using the function "addItem()", but when I try to call this same function from another file I get the error 

Thread 1: EXCBADINSTRUCTION (code=EXCI386INVOP, subcode=0x0) 

on the line 

newItem.order = (listItems.last?.order ?? 0) + 1.

Code Block 
var body: some View {
		NavigationView {
				VStack {
						List {
								ForEach(listItems, id: \.self) { item in
										NavigationLink(destination: ListItemView(listItem: item)) {
												Text("\(item.name) - \(item.order)")
										}
								}
								.onDelete(perform: deleteItem)
								.onMove(perform: moveItem)
						}
					 // Button(action: addItem) {
						//		Text("Add Item")
					 // }
						Button(action: {
								self.isShowingAlert.toggle()
						}) {
								Text("Add Item")
						}
				}
				.navigationBarItems(trailing: EditButton())
		}
		.addSujectPopUp(isShowing: $isShowingAlert, text: $alertInput, title: "Add Subject")
}
func moveItem(indexSet: IndexSet, destination: Int) {
		let source = indexSet.first!
		
		if source < destination {
				var startIndex = source + 1
				let endIndex = destination - 1
				var startOrder = listItems[source].order
				while startIndex <= endIndex {
						listItems[startIndex].order = startOrder
						startOrder = startOrder + 1
						startIndex = startIndex + 1
				}
				
				listItems[source].order = startOrder
				
		} else if destination < source {
				var startIndex = destination
				let endIndex = source - 1
				var startOrder = listItems[destination].order + 1
				let newOrder = listItems[destination].order
				while startIndex <= endIndex {
						listItems[startIndex].order = startOrder
						startOrder = startOrder + 1
						startIndex = startIndex + 1
				}
				listItems[source].order = newOrder
		}
		
		saveItems()
}
func deleteItem(indexSet: IndexSet) {
		let source = indexSet.first!
		let listItem = listItems[source]
		managedObjectContext.delete(listItem)
		saveItems()
}
func addItem() {
		let newItem = ListItem(context: managedObjectContext)
		newItem.name = alertInput
		newItem.order = (listItems.last?.order ?? 0) + 1
		saveItems()
}
func saveItems() {
		do {
				try managedObjectContext.save()
		} catch {
				print(error)
		}
}

Pop Up Code button:

Code Block 
let subject = SubjectController()
Button(action: self.subject.addItem) {
														Text("Add")
												}