iPhone app crashes when user exits UITableView with MBProgressHUD running in separate thread

In my app, I have a UITableViewController which loads calculated data from my Core Data store. This can take quite some time and hangs the main thread, so to give the user visual feedback, I have installed the MBProgressHUD widget which will show a progress indicator (just the spinning wheel) while the calculation method runs in a separate thread.

This is fine, except that the user can still exit the UITableViewController if they think it's taking too long. Of course, this is a good thing, except that when the separate thread concludes its operation, it still tries to call its delegate (the UITableViewController) to hide the MBProgressHUD. This causes a crash because since the user already exited the UITableViewController, it has dealloc'd and release'd itself.

MBProgressHUD has the following code to try to stop this:

if(delegate != nil && [delegate conformsToProtocol:@protocol(MBProgressHUDDelegate)]) {
    if([delegate respondsToSelector:@selector(hudWasHidden)]) {
        [delegate performSelector:@selector(hudWasHidden)];
    }
}

However, my app somehow seems to still be running this inner code ([delegate performSelector:@selector(hudWasHidden)]) even though the UITableViewController is totally gone--causing the app to crash.

Any suggestions? I am not running with NSZombiesEnabled.