How to debug "precondition failure" in Xcode?

I had a TabView containing a view that used a List.When switching tabs, my app was crashing with a similar error:"precondition failure: attribute failed to set an initial value: 99"This crashed:

var body: some View {    TabView {        ListView()        .tabItem {            Image(systemName: "list.dash")            Text("List")        }

Wrapping the ListView in a NavigationView fixed the crash.I saw this use of NavigationView on "Swift Live – 007 SwiftUI TabView && List" by Caleb Wells.https://youtu.be/v1A1H1cQowI

https://github.com/calebrwells/A-Swiftly-Tilting-Planet/tree/master/2019/Live%20Streams/TabView%20List

This worked:

var body: some View {    TabView {        NavigationView { ListView() }        .tabItem {            Image(systemName: "list.dash")            Text("List")        }

I've run into this as well. I just want to share it in case someone finds it useful.

SHORT ANSWER

Wrapping my view into a NavigationView would raise the error. Using .navigationViewStyle(StackNavigationViewStyle()) solved my problem.

LONG ANSWER

I had something like this:

NavigationView {    GeometryReader { proxy in        VStack {            Text("Dummy")            Spacer()                        MyView()    // CONTAINS HAS A GEOMETRY READER TOO                .frame(width: min(proxy.size.width, proxy.size.height),                       height: min(proxy.size.width, proxy.size.height))             Spacer()            Text("Dummy")        }    }}

And then, MyView had a GeometryReader inside too. The code as described would fail. If NavigationView was removed, the precondition failure wouldn't happen.

I used .navigationViewStyle(StackNavigationViewStyle()) on NavigationView and that solved my issue.

I've had this runtime error on simulators. In my case the problem was NavigationBarItems, I used it inside a wrong block as below:

NavigationView {    Group {        if something {                        ScrollView {                ...            }//ScrollView                    } else {            ...        }            }//group        .navigationBarItems(trailing: self.favoriteItem) // CRASH**}

I moved the NavigationBarItems modifier and gave it to ScrollView:

NavigationView {        Group {            if something {                        ScrollView {                ...            }//ScrollView            .navigationBarItems(trailing: self.favoriteItem) // NO CRASH**                    } else {            ...        }            }//group}