iOS - pushViewController not working after conversion to xCode 9 / iOS 11 - ios

Question

I saw my iOS app isn't allowed to be downloaded from the app store with the most recent upgrade to iOS 11, so I'm trying to update it. I have it in Xcode 9, and I've managed to now get the app started on my iPhone 6. However, I can't seem to get it to display after my initial screen display.
For example, the initial display successfully displays and shows a button called "start". That should pop up another display when you press it, but it doesn't (although it is reaching the code which displays it).
Here's the code, in a class named "StartController.m", which should be calling up the display:
NSLog(#"nav controller = %#", self.navigationController);
// push question controller onto the stack
[self.navigationController pushViewController:questionController animated:YES];
// the nav controller owns it now, we can release it
[questionController release];
Debugging shows this code is successfully called when the button is pressed, but after the release of questionController, nothing happens.
Here's the declaration of the questionController in StartController.h:
#interface StartController : UIViewController <ADBannerViewDelegate>{
QuestionController *questionController;
And here's the declaration of the same in StartController.m:
#implementation StartController
#synthesize questionController, settingsController, nextLevelViewController, appState;
The initial view (with the "start" button) is successfully displayed with this code in the "didFinishLaunchingWithOptions" method of the app delegate:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
// Override point for customization after application launch.
// create the start controller
self.startController = [[StartController alloc] init];
// transfer to it - not animated because it's the first view
[navController pushViewController:startController animated:NO];
// add the current nav controller view to the window view heirarchy
// [window addSubview:navController.view];
[self.window setRootViewController:startController];
// release becasause now the navController has it
[startController release];
// show the start controller
[self.window makeKeyAndVisible];
return YES;
}
For reference, here's the declaration of the navController in the app delegate header:
#interface QuizAppDelegate : NSObject <UIApplicationDelegate> {
UIWindow *window;
UINavigationController *navController;
StartController *startController;
}
#property (readonly) BOOL iPad;
#property (nonatomic, retain) IBOutlet UIWindow *window;
#property (nonatomic, retain) StartController *startController;
#property (nonatomic, retain) IBOutlet UINavigationController *navController;
And here's the declaration in the implementation:
#implementation QuizAppDelegate
#synthesize window;
#synthesize startController;
#synthesize navController;
I tried not to post too much code for fear of obscuring the problem, but I'll add it if requested. Any ideas what the problem might be?