SwiftUI, NavigationLink and UndoManager

I'm trying to access UndoManager via Environment somewhere deep in the navigation hierarchy:

import SwiftUI

@main
struct TestUndoManagerEnvironmentApp: App {
  var body: some Scene {
    DocumentGroup(newDocument: TestUndoManagerEnvironmentDocument()) { file in
      WrapperContent()
    }
  }
}

struct WrapperContent: View {
  @Environment(\.undoManager) private var undoManager: UndoManager!

  var body: some View {
    NavigationLink(destination: ContentView()) {
      Text("go")
    }
    Text("\(undoManager.canRedo ? "t" : "f")")
  }
}

struct ContentView: View {
  @Environment(\.undoManager) private var undoManager: UndoManager!

  var body: some View {
    Text("\(undoManager.canRedo ? "t" : "f")")
  }
}

And whenever I run the app I get the Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value in ContentView.body.

Am I doing something clearly wrong?