警告:试图呈现的视图上不是在层次结构的窗口!

Warning: Attempt to present <CallViewController: 0x1281cd200> on <MainControllerViewController: 0x127833800> whose view is not in the window hierarchy!

//可以获取当前的controller（最上方的controller）

• (UIViewController*)topViewController
  {
  return [self topViewControllerWithRootViewController:self.window.rootViewController];
  }

• (UIViewController)topViewControllerWithRootViewController:(UIViewController)rootViewController
  {
  if ([rootViewController isKindOfClass:[UITabBarController class]]) {
  UITabBarController tabBarController = (UITabBarController )rootViewController;
  return [self topViewControllerWithRootViewController:tabBarController.selectedViewController];
  } else if ([rootViewController isKindOfClass:[UINavigationController class]]) {
  UINavigationController navigationController = (UINavigationController)rootViewController;
  return [self topViewControllerWithRootViewController:navigationController.visibleViewController];
  } else if (rootViewController.presentedViewController) {
  UIViewController* presentedViewController = rootViewController.presentedViewController;
  return [self topViewControllerWithRootViewController:presentedViewController];
  } else {
  return rootViewController;
  }
  }