Why cant I see my for Each loop in the preview view in XCode - xcode

Question

I am making a to do list to learn about Core data. I noticed when I add a ForEach loop to read my core data entries, it stops loading the view and the app crashes. If I replace the ForEach loop with a Text("Hi"), it all works. However the app runs perfectly in simulator.
import SwiftUI
struct ContentView: View {
#Environment(\.managedObjectContext) var managedObjectContext
#FetchRequest(fetchRequest: ToDoItem.getAllToDo()) var toDoItems:FetchedResults<ToDoItem>
#State private var newTodoItem = ""
#State private var selection = 0
var body: some View {
NavigationView {
List {
Section (header: Text("Whats... next")) {
HStack{
TextField("New Item", text: self.$newTodoItem)
Button(action: {
let toDoItem = ToDoItem(context: self.managedObjectContext)
toDoItem.title = self.newTodoItem
toDoItem.createdAt = Date()
do {
try self.managedObjectContext.save()
}catch {
print(error)
}
self.newTodoItem = ""
}){
Image(systemName: "plus.circle.fill")
.foregroundColor(.green)
.imageScale(.large)
}
}
}.font(.headline)
Section(header: Text("To Do's")){
ForEach(self.toDoItems) {todoItem in
HStack{
VStack(alignment: .leading){
Text(todoItem.title!)
.font(.headline)
Text("\(todoItem.createdAt!)")
.font(.caption)
}
}
}.onDelete {indexSet in
let deleteItem = self.toDoItems[indexSet.first!]
self.managedObjectContext.delete(deleteItem)
do {
try self.managedObjectContext.save()
}catch {
print(error)
}
}
}
}
.navigationBarTitle(Text("My List"))
.navigationBarItems(trailing: EditButton())
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
Any idea what the issue is?