onAppear is causing problem with the preview but no error is shown - core-data

Question

self learning beginner here.
When I remove .onAppear{add()}, the preview works fine. I tried to attach it to other the body view, the Vstack but it causes another error. I read/watched several tutorials but nothing like this is mentioned....
Any help is appreciated
struct ListView: View {
#Environment(\.managedObjectContext) var moc
#FetchRequest(sortDescriptors: []) var targets: FetchedResults<TargetEntity>
#FetchRequest(sortDescriptors: []) var positives: FetchedResults<PositiveEntity>
var body: some View {
VStack {
Text("+")
.onAppear{add()}
.onTapGesture (count: 2){
do {
increment(targets.first!) //I also sense that doing "!" is not good. But it's the only way I can keep it from causing error "Cannot convert value of type 'FetchedResults' to expected argument type 'X'"
try moc.save()
} catch {
print("error")
}
}
}
}
func increment(_ item: TargetEntity) {
item.countnum += 1
save()
}
func add() {
let countnum = TargetEntity(context: moc)
countnum.countnum = 0
save()
}
func save() {
do { try moc.save() } catch { print(error) }
}
}
EDIT 20220509:
As advised by #Yrb (great thanks), the error is likely caused by the lack of a proper set up of preview var in the persistence file. I post the relevant code here for visiblity.
Data Controller file
import CoreData
import Foundation
class DataController: ObservableObject {
let container = NSPersistentContainer(name: "CounterLateApr")
init () {
container.loadPersistentStores { description, error in
if let error = error {
print("Core Data failed to load: \(error.localizedDescription)")
}
}
}
}
preview code in a view
struct ListView_Previews: PreviewProvider {
static var previews: some View {
NavigationView{
ListView()
}
}
}
[AppName].app file
import SwiftUI
#main
struct CounterLateAprApp: App {
#StateObject private var dataController = DataController()
var body: some Scene {
WindowGroup {
ContentView()
.environment(\.managedObjectContext, dataController.container.viewContext)
}
}
}