message: Added defensive checks
  Have seen them fall over when testing overlapping. Not sure of the circumstances so can't recreate reliably but no harm in putting checks in to prevent errors
diff: - (void)checkPeekability:(NSInteger)crumb
  {
  NVSceneView *scene;
  - if (crumb > 1) {
  + if (crumb > 1 && self.keys.count > crumb - 1) {
  scene = (NVSceneView *) [_scenes objectForKey:[self.keys objectAtIndex:crumb - 1]];
  }
  _navigationController.interactivePopGestureRecognizer.enabled = scene ? scene.subviews.count > 0 : YES;
  - (BOOL)navigationBar:(UINavigationBar *)navigationBar shouldPopItem:(UINavigationItem *)item
  {
  + NVSceneView *scene;
  NSInteger crumb = [[navigationBar items] indexOfObject:item];
  - NVSceneView *scene = ((NVSceneView *) [self.viewControllers objectAtIndex:crumb - 1].view);
  - return scene.subviews.count > 0;
  + if (self.viewControllers.count > crumb - 1)
  + scene = ((NVSceneView *) [self.viewControllers objectAtIndex:crumb - 1].view);
  + return scene ? scene.subviews.count > 0 : YES;
  }
  @end
  