Untraceable SIGABRT: precondition failure: attribute failed to set an initial value: <num>

Edit: Small changes to posted code to allow easier testing in tmp project. Create a new Single View App that using SwiftUI. C&P View A into ContentView.swift, and View B into a new files called "Dots"

I have also found that the issue does not occur when passing 0 to Dots, and have added that to the picker to aid testing.

I have spent countless hours doing my best to solve this. Read through too many other posts about this issue being caused by a lot of different things, hacked and slashed code and simple print statement/lldb to try track it down and I'm getting now - I'm pretty frantic right now.

This is the same core problem as a previous post

View works fine until used in NavigationView/Link

Whilst the workaround stopped the crashes, it was definitely a workaround and not fixing the source. As a consequence, a different "bug" appeared why a custom view partially ignored safe boundaries*. Rather than trying to find a hacky workaround for the workaround, it would be far better to try solve the core issue, but am deeply struggling.

As explained in the previous post, View A and B work fine on their own, but when navigation from A to B, that when it fails. Stepping through the code didn't yield much but it be a case of not knowing what to look for.

* this view consisted of x views that behaved like horizontal cards, swiping left/right to jump between them. This was achieved by having a HStack(spacing: 0) with a frame x times the width of the screen and a custom Gesture. If you envisage the views as an array, index 0 would be shown by default, and one can swipe amongst them. The first 2 items would correctly respect the inherited height, but 3 onwards would go behind the navigation space and status bar.

Note I've attempted to clear the code of some unnecessary content so as to avoid wasting your time.

View A (Note I've attempted to clear the code of some unnecessary content so as to avoid wasting your time.)

import SwiftUI
  
let lowerBound = 0
let upperBound = 101

struct CapsuleButton: ViewModifier {
    func body(content: Content) -> some View {
        content
            .frame(width: 250, height: 100)
            .background(Capsule().fill(Color.blue))
            .foregroundColor(.black)
            .font(.system(size: 28, weight: .light))
            .padding()
    }
}

struct ContentView: View {
    @State private var pickerIndex: Int = UserDefaults.standard.integer(forKey: "highestDotCovered") - lowerBound
    @State private var nav: Int? = 0
      
    var body: some View {
        VStack(spacing: 30) {
            HStack {
                Picker(selection: $pickerIndex, label: Text("")) {
                    ForEach(lowerBound..<upperBound) { Text(String($0)) }
                }
            }
            Spacer()
            NavigationLink(destination: Dots(lowerBound + pickerIndex), tag: 1, selection: $nav) { EmptyView() }
  
            Text("Continue")
                .modifier(CapsuleButton()) // custom look
                .onTapGesture {
                    // Update storage
                    let currentHighest = UserDefaults.standard.integer(forKey: "highestDotCovered")
                    let highest = max(lowerBound + self.pickerIndex, currentHighest)
                    UserDefaults.standard.set(highest, forKey: "highestDotCovered")
                    // Go to destination
                    self.nav = 1
                }
        }
        .navigationBarTitle("Quantity Selection")
    }
}
  
struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        NavigationView {
            ContentView()
        }.navigationViewStyle(StackNavigationViewStyle())
    }
}

View B

import SwiftUI
  
private let diam: CGFloat = 60 // ~1/3", 1pt = 1/163"
private let maxPhoneDots = 12
  
struct Dots: View {
    @State private var dotAlert = false
      
    let quantity: Int
    let dot: some View = Circle()
        .size(width: diam, height: diam)
        .fill(Color.red)
    @State var offsets: [CGSize] = []
      
    init(_ quantity: Int) {
        let model = UIDevice().model
        self.quantity = model == "iPad" ? quantity : min(quantity, maxPhoneDots)
        print(self.quantity)
    }
  
    var body: some View {
//        NavigationView { // need to wrap geom reader in nav view to avoid crash
//            VStack {
                GeometryReader { geom in
                    ForEach(self.genOffsets(maxWidth: geom.size.width, maxHeight: geom.size.height), id: \.self) {offset in
                        self.dot
                            .offset(offset)
                    }
                    .background(Color.clear
                        .contentShape(Rectangle())) // workaround for allowing gestures on clear background
                }
                .border(Color.black)
                  
                //            Spacer()
//                Text("Dots: \(self.quantity)")
//            }
//            .navigationBarTitle("")
//            .navigationBarHidden(true)
//        }.navigationViewStyle(StackNavigationViewStyle())
    }
}
  
extension Dots {
    func genOffsets(maxWidth: CGFloat, maxHeight: CGFloat) -> [CGSize] {
        var cols = UIDevice().model == "iPad" ? 8 : 4
        var rows = UIDevice().model == "iPad" ? 13 : 5
        if UIDevice().orientation.isLandscape {
            (cols, rows) = (rows, cols) //swap cols and rows
        }
          
        var offsets: [CGSize] = []
  
        let availableWidth = max(maxWidth - (CGFloat(cols) * diam), 0)
        let colSeparation: CGFloat = availableWidth / CGFloat((cols+1))
        let availableHeight = max(maxHeight - (CGFloat(rows) * diam), 0)
        let rowSeparation: CGFloat = availableHeight / CGFloat((rows+1))
          
        let xJitter = colSeparation * 0.49
        let yJitter = rowSeparation * 0.49
  
        // grid[row][col]
        var grid = Array(repeating: Array(repeating: false, count: cols), count: rows)
  
        // Generate dot offsets
        for _ in 0..<quantity {
            var r, c: Int
            // Find a random empty cell
            repeat {
                r = Int.random(in: 0..<rows)
                c = Int.random(in: 0..<cols)
            } while grid[r][c]
            grid[r][c] = true
  
            // Calculate dot's offset
            //      Initial padding from TL corner
            var x = colSeparation
            var y = rowSeparation
            //      Separate dot into standard grid slot
            x += CGFloat(c)*(diam+colSeparation)
            y += CGFloat(r)*(diam+rowSeparation)
            //      Apply jitter
            if xJitter != 0 { x += CGFloat.random(in: -xJitter ..< xJitter) }
            if yJitter != 0 { y += CGFloat.random(in: -yJitter ..< yJitter) }
  
            offsets.append(CGSize(width: x, height: y))
        }
          
        return offsets
    }
}
  
extension CGSize: Hashable {
    public func hash(into hasher: inout Hasher) {
      hasher.combine(width)
      hasher.combine(height)
    }
}

struct Dots_Previews: PreviewProvider {
    static var previews: some View {
        Dots(1)
    }
}