SwiftUI - Navigation bar button issue workaround.

ContentView.swift

import SwiftUI

struct ContentView: View {
  
    // If the next line is commented, the button will become not clickable after the presentation.
    @Environment(\.presentationMode) var presentation
    @State var showSheet = false

    var body: some View {
        NavigationView {
            VStack {
                Text("Test")
            }.sheet(isPresented: self.$showSheet) {
                ModalView()
            }.navigationBarItems(trailing:
                Button(action: {
                    self.showSheet = true
                }) {
                    Text("SecondView")
                }
            )
        }
    }
}

struct ModalView: View {

    @Environment(\.presentationMode) var presentation

    var body: some View {
        VStack {
            Button(action: {
                self.presentation.wrappedValue.dismiss()
            }) {
                Text("Dismiss")
            }
        }
    }
}

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