UIViewControllerHierarchyInconsistency

I am trying to build my app and at one point I push a UIViewController and then I get this error. I am not exactly sure why.

'UIViewControllerHierarchyInconsistency', reason: 'A view can only be associated with at most one view controller at a time! View > is associated with . Clear this association before associating this view with .'

PageViewController *viewController;

viewController = [[PageViewController alloc] initWithManagedObjectContext:managedObjectContext];
dataSource = [[PagesDataSource alloc] initWithManagedObjectContext:managedObjectContext];

PVPage *selectedPage = [[dataSource pages] objectAtIndex:itemIndex];
[viewController setRepresentedPage:selectedPage];

PageFlipperAppDelegate *appDelegate = (PageFlipperAppDelegate *)[[UIApplication sharedApplication] delegate];
[(UINavigationController *)[[appDelegate window] rootViewController] setToolbarHidden:YES animated:YES];
[(UINavigationController *)[[appDelegate window] rootViewController] pushViewController:viewController animated:YES];

In my pageViewController...................

- (id)initWithManagedObjectContext:(NSManagedObjectContext *)initManagedObjectContext
{
    if ((self = [super initWithNibName:@"PageView" bundle:nil]))
    {
        [self setManagedObjectContext:initManagedObjectContext];
        dataSource = [[PagesDataSource alloc] initWithManagedObjectContext:[self managedObjectContext]];
    }
    return self;
}