title: SwiftUI app work in preview but not in simulator
body: <p>I'm making an app for a school project where I try to calculate the weight of a person on different planets. The  ContentView file works fine in the preview, but only shows NavigationBar.title in the simulator / iPhone. Here is the code </p>
  
  <pre><code>import SwiftUI
  
  struct ContentView : View {
      var planets: [Planet] = []
      var body: some View {
          NavigationView {
  
              List(planets) { planet in
                  PlanetCell(planet: planet)
              }
              .navigationBarTitle(Text("Sun"))
  
          }
      }
  }
  
  #if DEBUG
  struct ContentView_Previews : PreviewProvider {
      static var previews: some View {
          ContentView(planets: testData)
      }
  }
  #endif
  
  struct PlanetCell : View {
      let planet : Planet
      var body: some View {
          return NavigationLink(destination: PlanetDetail(planet: planet)){
              Text(planet.name)
          }
      }
  }
  
  </code></pre>
  
  <p>edit- swift file called Planet</p>
  
  <pre><code>import Foundation
  import SwiftUI
  struct Planet : Identifiable {
      var id = UUID()
      var name: String
      var g : Float
      var imageName : String { return name }
  }
  
  #if DEBUG
  let testData = [
  
      Planet(name: "Mercury", g: 3.724),
      Planet(name: "Venus", g: 8.918),
      Planet(name: "Earth", g: 9.8),
      Planet(name: "Mars", g: 3.724),
      Planet(name: "Jupiter", g: 22.932),
      Planet(name: "Saturn", g: 9.114),
      Planet(name: "Uranus", g: 9.016),
      Planet(name: "Neptune", g: 10.976)
  
  ]
  
  #endif
  
  
  </code></pre>