I've got the following error when saving a new activity, and can't get it fixed.

"Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)"

Here's my code:

struct Activity: Identifiable, Equatable, Hashable
{
    let id = UUID()
    var title: String
    var description: String
    var count: Int = 0
}

class Activities: ObservableObject
{
    @Published var items = [Activity]()
}

struct ContentView: View {
    @ObservedObject var activities = Activities()

    @State private var showingNewActivityScreen = false

    var body: some View {
        NavigationView {
            VStack {
                ForEach(activities.items) { activity in
                    HStack {
                        Text("\(activity.title)")
                            .frame(minWidth: .infinity)
                            .padding()
                            .background(Color.blue)
                            .foregroundColor(Color.white)
                            .clipShape(RoundedRectangle(cornerRadius: 16))

                        Spacer()

                        Text("\(activity.count)").padding(.horizontal)
                        Button(action: {
                            let currentTrackerIndex = self.activities.items.firstIndex { $0 == activity }
                            if let index = currentTrackerIndex { self.activities.items[index].count += 1 }
                        }) { Image(systemName: "plus").padding() }
                    }
                }
            }
            .navigationBarTitle("Habit Tracking")
            .navigationBarItems(trailing:
                Button(action: { self.showingNewActivityScreen = true },
                       label: { Text("New") } )
            )
        }
        .sheet(isPresented: $showingNewActivityScreen, content: { RegisterActivity(activities: self.activities, "New activity") })
    }
}

struct RegisterActivity: View {
    @ObservedObject var activity: Activities
    @Environment(\.presentationMode) var presentationMode

    @State private var title = ""
    @State private var description = ""

    let screenTitle: String

    var body: some View {
        VStack {
            HStack {
                Text("Title")
                TextField("Title", text: $title)
            }.padding()
            HStack {
                Text("Description")
                TextField("Description", text: $description)
            }.padding()
            HStack {
                Button(action: {
                    self.presentationMode.wrappedValue.dismiss()
                }) {
                    Text("Cancel").padding()
                }
                Spacer()
                Button(action: {
                    self.activity.items.append(Activity(title: self.title, description: self.description))
                    //print("\(self.trackers.items)")
                    self.presentationMode.wrappedValue.dismiss()
                }) {
                    Text("Save").padding()
                }
            }
        }.navigationBarTitle("New Activity")
    }

    init(activities: Activities, _ screenTitle: String = "New Item")
    {
        self.activity = activities
        self.screenTitle = screenTitle
    }
}

I can't spot what's wrong. Something in the ForEach maybe? Is it how I use the Observable/Observed thing? Any clues?

Thank you @delawaremathguy. This is the exact solution I got at the slack channel for this forum (including the VStack vs List thing 😂)