How can I stop this UINavigationController from releasing view controllers - objective-c

Question

I have an app which comprises of 2 main controller types:
1) A UITableViewController - which acts as a navigation screen
2) A UIViewController contained within a UINavigationController - which shows the main app content.
The TableViewController content looks a little like this:
Page 1
Page 2
Page 3
... etc
I've defined a property on AppDelegate - an array called PageViewControllers.
When the application starts, a new instance of UIViewController is created for each Page within the app.
The first Page's controller is set as the UINavigationController's rootViewController.
When the user selects a row in the UITableView controller, the UINavigationController pushes or pops to the relevant view controller for that row. (If I tap "Page 3", it pushes to the controller for Page three).
I have this working well - the only problem is the app occasionally crashes when trying to jump far back in the navigation stack. For example, from Page 15 to Page 2
The error message I'm getting is:
*** Terminating app due to uncaught exception 'RuntimeError', reason: 'NSInternalInconsistencyException: Tried to pop to a view controller that doesn't exist.
I think the UINavigationController might be releasing some controllers. I thought the app would hold all of the previous controllers in memory to allow the UINavigationController's back button to behave as expected?
Any idea how I can prevent this from happening or is there something I've missed?
Update
Here's the code where I push/pop the navigation controller after a table row is selected. (It's Rubymotion)
def tableView(tableView, didSelectRowAtIndexPath: indexPath)
# first we need to work out which controller was selected...
page = Page.current
currentPageController = appDelegate.pageControllers[page.absoluteIndex]
currentPageControllerIndex = appDelegate.pageControllers.index(currentPageController)
nextPageController = appDelegate.pageControllers[Page.pageAtIndexPath(indexPath).absoluteIndex]
nextPageControllerIndex = appDelegate.pageControllers.index(nextPageController)
case
# When we're moving forward in the stack...
when currentPageControllerIndex < nextPageControllerIndex
for controller in appDelegate.pageControllers[(currentPageControllerIndex + 1)..nextPageControllerIndex]
# push the next controller on to the nav stack, only animate if it's the last one
appDelegate.rootViewControllerNav.pushViewController(controller, animated: controller == nextPageController)
end
# When we're moving backward in the stack...
when currentPageControllerIndex > nextPageControllerIndex
appDelegate.rootViewControllerNav.popToViewController(nextPageController, animated: true)
# When we select the same step again...
else
NSLog("Selected the same page")
end
# close the side menu afterwards
appDelegate.rootViewControllerNav.sideMenu.toggleSideMenuPressed(self)
end

Answer 1

"I thought the app would hold all of the previous controllers in memory to allow the UINavigationController's back button to behave as expected?"
As long as you're going forward this is true -- as you push a new view controllers, it adds the new one to the previous ones on the stack. But, when you popViewController, it's removed from the stack, and if you don't have a strong reference to it, it will be deallocated. So if you go 1-->2-->3........-->15 and then pop to 2, it should work, but if you do something like this:
1-->2-->3 popTo 1 then push to-->15 then pop to 2, neither 2 nor 3 will exist any more.
So, you should pay attention to what sequence you're picking cells in your table, and note when you get the error. See if it corresponds to this explanation.

Answer 2

Cracked it!
The problem here was in the logic elsewhere in my app. You'll see that page is set from Page.currentPage.
page = Page.currentPage
Page.currentPage should always be set to the page that's currently being displayed.
The code to set current page was in the viewDidLoad method of the PagesViewController instead of the viewDidAppear:animated method - meaning that one a page had been viewed, it wouldn't set Page.currentPage again.
Fixed now - thanks a lot for the ideas and tips shared!