message: Prevented error navigating back over fluent scene
  Navigate fluently from A --> B --> C then navigate back 2 to A. The NativeViewHierarchyManager throws layout error because the child Scene is null. Only clean up removed scenes if they were visited, had a child view
diff: @@ -73,8 +73,11 @@ public class NavigationStackView extends ViewGroup {
  @Override
  public void removeViewAt(int index) {
  - if (index < sceneItems.size())
  - sceneDiscards.add(sceneItems.remove(index).view);
  + if (index < sceneItems.size()) {
  + SceneView view = sceneItems.remove(index).view;
  + if (view.getChildCount() > 0)
  + sceneDiscards.add(view);
  + }
  }
  protected void onAfterUpdateTransaction() {
  