Dismissing modal view controller results in a black screen

This is my view (controller) hierarchy:

• UITabBarController (as the rootViewController of the app)
• UINavigationController (as the viewController for one of the tabBar tabs)
• UIViewController (as the rootViewController of the UINavigationController)
• UICollectionView (as a subview)
• MyViewController.view (as a section header view of the UICollectionView)

And so, I need to present a modal view controller from MyViewController. I have tried doing it with

[self presentViewController:modalVC animated:YES completion:nil];

and although it worked, Xcode warned me that "Presenting view controllers on detached view controllers is discouraged", and rightly so, because the modalVC only fills the view of the collection view header, which is not full screen which I'm after.

All other options that I have tried:

UITabBarController *tb = (UITabBarController *)self.view.window.rootViewController;
[tb presentViewController:modalVC animated:YES completion:nil];

or...

UINavigationController *nc = (UINavigationController *)tb.selectedViewController;
[tb presentViewController:modalVC animated:YES completion:nil];

or...

UICustomViewController *cv = (UICustomViewController *)nc.topViewController;
[vc presentViewController:modalVC animated:YES completion:nil];

present the modalVC full screen as desired, however, when I dismiss the modalVC by calling

[self dismissViewControllerAnimated:YES completion:nil];

from within modalVC itself, modalVC indeed dismisses itself, but I am left with a black screen. A little debugging revealed that after dismissing modalVC, self.view.window.rootViewController becomes nil.

Any idea why this is happening and how to resolve this?

EDIT

This is an iPhone app. The black screen happens on both iOS7 and iOS8. Also, below is the method where I initiate MyViewController

#pragma mark - UICollectionViewDelegate methods

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section {

    self.myViewController = [[MyViewController alloc] initWithNibName:NSStringFromClass([MyViewController class]) bundle:nil];

    return self.myViewController.view.frame.size;
}

I found the solution - this answer really helped. The trick was in dismissing the view controller. It should be done like this:

[self.presentingViewController dismissViewControllerAnimated:YES completion:nil];

and not like this:

[self dismissViewControllerAnimated:YES completion:nil];

Although the author of the linked answer suggests that a better approach would be to use delegation (presentED VC would define a protocol and presentING VC would subscribe to it and then dismiss the presentED VC once it asks for it), it wasn't feasible in my case.