when my custom alert show, navigation bar back button still can press

it's really a simple demo here, when I use NavigationLink destination, go to ContentView3(),i show a custom alert, when it show up, the navigation back button still work.what can I do, when my alert show , and set the back button clickable to false?(the alert must be a custom one, cannot use system .alert)

import SwiftUI

struct ContentView: View {
    @State var isActive : Bool = false

    var body: some View {
        NavigationView {
            NavigationLink(
                destination: ContentView2(rootIsActive: self.$isActive),
                isActive: self.$isActive
            ) {
                Text("Hello, World!")
            }
            .isDetailLink(false)
            .navigationBarTitle("Root")
        }.navigationViewStyle(.stack)
    }
}

struct ContentView2: View {
    @Binding var rootIsActive : Bool

    var body: some View {
        NavigationLink(destination: ContentView3(shouldPopToRootView: self.$rootIsActive)) {
            Text("Hello, World #2!")
        }
        .isDetailLink(false)
        .navigationBarTitle("Two")
    }
}

struct ContentView3: View {
    @Binding var shouldPopToRootView : Bool
    @State var showTip = false

    var body: some View {
        ZStack {
            VStack {
                Text("Hello, World #3!")
                Button (action: {
                    self.shouldPopToRootView = false }
                )
                {
                    Text("Pop to root")
                }
                Button (action: {
                    self.showTip.toggle() }
                )
                {
                    Text("show tip")
                }
            }
// when alert show up, the back button still work, what can I do stop it? let the back button clickable to false?
            if showTip {
                TipAlert(isPresented: $showTip, tip: "123445", negative: "", positive: "OK") {
                    
                }
            }
        }
        .navigationBarTitle("Three")
    }
}

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

struct TipAlert: View {
    @Binding var isPresented: Bool
    let tip: String
    let negative: String
    let positive: String
    let action: (() -> Void)
    var body: some View {
        ZStack {
            Color.gray.edgesIgnoringSafeArea(.all).opacity(0.75)
            VStack(spacing: 10) {
                Text(tip)
                    .font(.system(size: kFrontSize_18))
                    .foregroundColor(.black)
                HStack (alignment: .center, spacing: 20){
                    Spacer()
                    if !negative.isEmpty {
                        Button {
                            isPresented.toggle()
                        } label: {
                            Text(negative)
                                .font(.system(size: kFrontSize_15))
                                .padding(.horizontal, 12)
                                .padding(.vertical, 8)
                                .frame(minWidth: 55)
                                .foregroundColor(.white)
                                .clipShape(RoundedRectangle(cornerRadius: 5))
                        }
                    }
                    Button {
                        action()
                        isPresented.toggle()
                    } label: {
                        Text(positive)
                            .font(.system(size: kFrontSize_15))
                            .padding(.horizontal, 12)
                            .padding(.vertical, 8)
                            .frame(minWidth: 55)
                            .foregroundColor(.white)
                            .clipShape(RoundedRectangle(cornerRadius: 5))
                    }
                    Spacer()
                }
            }
            .padding(10)
            .background(.white)
            .frame(
                width: 340
            )
            .cornerRadius(5)
        }
    }
}