I don't know how to fix this. I don't know how to code #preview part of EventView. Here is my code:

import SwiftUI

struct AddEvent: Codable, Identifiable {
    var id = UUID()
    let eventName: String
    let amount: Int
}

@Observable
class Events {
    var items = [AddEvent]() {
        didSet {
            if let encoded = try? JSONEncoder().encode(items) {
                UserDefaults.standard.set(encoded, forKey: "Items")
            }
        }
    }

    init() {
        if let savedItems = UserDefaults.standard.data(forKey: "Items") {
            if let decodedItems = try? JSONDecoder().decode([AddEvent].self, from: savedItems) {
                items = decodedItems
                return
            }
        }

        items = []
    }
}

let students = Bundle.main.decode("students.json")

struct ContentView: View {
    @State var events = Events()
    @State private var showingAddEvents = false

    var body: some View {
        NavigationStack {
            List {
                ForEach(events.items) { event in
                    NavigationLink {
                        EventView(event: event)
                    } label: {
                        HStack {
                            VStack {
                                Text(event.eventName)
                                Text("$ \(event.amount)")
                            }
                            Spacer()
                            Text("多少人沒繳錢")
                        }
                    }
                }
            }
            .navigationTitle("Payment Registration")
            .toolbar {
                Button("show add event", systemImage: "plus") {
                    showingAddEvents = true
                }
            }
            .sheet(isPresented: $showingAddEvents) {
                AddEventView(events: events)
            }
        }
    }
}

#Preview {
    ContentView()
}

import SwiftUI

struct EventView: View {
    let event: AddEvent

    var body: some View {
        let thisEventName = event.eventName

        NavigationStack {
            ForEach(students) {student in
                Text(student.name)
            }
            .navigationTitle(thisEventName)
        }
    }
}

#Preview {
    EventView()
}

Add

#Preview {
    EventView()
      .environment(Events())
}