SwiftUI + Core Data: unable to delete entity

I have created this TimerListView with (+) button on the toolbar. I'm able to create a new Timer entity using TimerEditView. But if in the TimerEditView I try to delete the entity clicking on "Dismiss" button, an error will be raised.

import SwiftUI
import CoreData


struct TimerListView: View {
    
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Timer.id, ascending: true)],
        animation: .default)
    private var timers: FetchedResults<Timer>
    
    @State private var isPresentingSheet = false
    @State private var showFavoritesOnly = false
    @State private var sheetView: SheetView = .setting
    @State var newTimerToAdd: Timer = Timer()

    
    
    var body: some View {
        
        NavigationView{
            
            List {
                
                ForEach(timers) { timer in
                    NavigationLink(destination: TimerDetailView(timer: timer)) {
                        TimerCardView(timer: timer)
                    }
                }

                
                
            }
            .navigationTitle("Timer")
            .toolbar {
                ToolbarItem(placement: .confirmationAction) {
                    Button(action: {
                        
                        
                        newTimerToAdd = Timer(context: viewContext)
                        newTimerToAdd.id = UUID()
                        newTimerToAdd.name = "Timer name"
                        newTimerToAdd.secondsOn = 0
                        newTimerToAdd.secondsOff = 0
                        newTimerToAdd.rounds = 1
                        newTimerToAdd.cycles = 1
                        newTimerToAdd.secondsOffBetweenCycles = 0
                        newTimerToAdd.favourite = false
                        
                        
                        sheetView = .newTimer
                        isPresentingSheet = true
                        
                        
                    }) {
                        Label("New timer", systemImage: "plus")
                    }
                }
            }
            
            
            .sheet(isPresented: $isPresentingSheet) {
                if (sheetView == .newTimer) {
                    NavigationView {
                        TimerEditView(timer: newTimerToAdd)
                            .toolbar {
                                ToolbarItem(placement: .cancellationAction) {
                                    Button("Dismiss") {

                                        viewContext.delete(newTimerToAdd)
                                        
                                        do {
                                            try viewContext.save()
                                        } catch {
                                            // Replace this implementation with code to handle the error appropriately.
                                            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                                            viewContext.rollback()
                                            //print("Failed to save context \(nsError.localizedDescription)")
                                            
                                            let nsError = error as NSError
                                            fatalError("CAZZO Unresolved error \(nsError), \(nsError.userInfo)")
                                            
                                            
                                        }
                                        
                                        isPresentingSheet = false

                                    }
                                }
                                ToolbarItem(placement: .confirmationAction) {
                                    Button("Add") {
                                        
                                       
                                        do {
                                            try viewContext.save()
                                        } catch {
                                            // Replace this implementation with code to handle the error appropriately.
                                            // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                                            let nsError = error as NSError
                                            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
                                        }
                                        
                                        //timers.append(newTimerToAdd)
                                        isPresentingSheet = false
                                        //newTimer = Timer()
                                    }
                                }
                            } // .toolbar
                    } // NavigationView
                } else {
                    NavigationView {
                        SettingsView()
                            .toolbar {
                                ToolbarItem(placement: .confirmationAction) {
                                    Button("Salva") {
                                        isPresentingSheet = false
                                    }
                                }
                            }
                    }
                }
            } //.sheet
             
        }

    }
}