I have encountered this warning:
pushViewController:animated: called on while an existing transition or presentation is occurring; the navigation stack
My Solution would be to call dismissViewControllerAnimated: first and then pop the viewcontroller from navigation stack this works for me :-
[self dismissViewControllerAnimated:false completion:nil];
[myNavigationControllerInstance popToRootViewControllerAnimated:true]; // myNavigationControllerInstance = Your Navigation Controller Instance