我创建了一个新的SwiftUI项目，但代码不会加载到实时预览窗口中，我每次都会收到以下错误：
无法在此文件中预览-。但当我在模拟器上运行它时，它也适用于其他视图。

这是密码

import SwiftUI
import FirebaseAuth

class AppViewModel: ObservableObject {

    let auth = Auth.auth()

    @Published var LoggedIn = false

    var isLoggedIn: Bool{
        return auth.currentUser != nil
    }

    func LogIn(email: String, password: String) {

        auth.signIn(withEmail: email, password: password) { [weak self] result, error in
            guard result != nil, error == nil else{
                return
            }

            DispatchQueue.main.async {
            // Success
            self?.LoggedIn = true
            }
        }

    }

    func SignUp(email: String, password: String) {

        auth.createUser(withEmail: email, password: password) { [weak self] result, error in
            guard result != nil, error == nil else{
                return
            }
            DispatchQueue.main.async {
            // Success
            self?.LoggedIn = true
            }
        }

    }
}

struct ContentView: View {
    @EnvironmentObject var ViewModel : AppViewModel
    var body: some View {
        NavigationView {
        if ViewModel.LoggedIn {
            Text ("You are Logged In")
            } else{
                LogInView()
        }
    }
        .onAppear {
            ViewModel.LoggedIn = ViewModel.isLoggedIn
        }
    }
}

struct LogInView: View {
    @State var email = ""
    @State var password = ""

    @EnvironmentObject var ViewModel : AppViewModel
    var body: some View {

        VStack {

            TextField("Email Adress", text: $email)
                .disableAutocorrection(true)
                .autocapitalization(.none)
                .padding()
                .background(Color(.secondarySystemBackground))
                .cornerRadius(15)

            SecureField("Password", text: $password)
                .disableAutocorrection(true)
                .autocapitalization(.none)
                .padding()
                .background(Color(.secondarySystemBackground))
                .cornerRadius(15)

            Button(action: {

                guard !email.isEmpty, !password.isEmpty else{
                    return
                }
                ViewModel.LogIn(email: email, password: password)
            }) {
                Text("Log In")
                    .foregroundColor(Color.white)
                    .font(.headline)
                    .frame(maxWidth: .infinity)
                    .frame(height: 50)
                    .background(Color.accentColor)
                    .cornerRadius(20.0)
            }

            NavigationLink("Create an account", destination: SignUpView())
                .padding()

        }
        .padding(.horizontal, 24.0)
        .navigationTitle("Welcome")
    }
}

struct SignUpView: View {
    @State var email = ""
    @State var password = ""

    @EnvironmentObject var ViewModel : AppViewModel
    var body: some View {

        VStack {

            TextField("Email Adress", text: $email)
                .disableAutocorrection(true)
                .autocapitalization(.none)
                .padding()
                .background(Color(.secondarySystemBackground))
                .cornerRadius(15)

            SecureField("Password", text: $password)
                .disableAutocorrection(true)
                .autocapitalization(.none)
                .padding()
                .background(Color(.secondarySystemBackground))
                .cornerRadius(15)

            Button(action: {

                guard !email.isEmpty, !password.isEmpty else{
                    return
                }
                ViewModel.SignUp(email: email, password: password)
            }) {
                Text("Sign Up")
                    .foregroundColor(Color.white)
                    .font(.headline)
                    .frame(maxWidth: .infinity)
                    .frame(height: 50)
                    .background(Color.accentColor)
                    .cornerRadius(20.0)
            }

        }
        .padding(.horizontal, 24.0)
        .navigationTitle("Create an Account")
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}