Seems that you are trying make mutable copy of nil in line
[self.navigationController.toolbar.items mutableCopy];
By default method navigationController.toolbar.items nas no items and returns nil
Update
Method - (void)setToolbarItems:(NSArray*)toolbarItems animated:(BOOL)animated does nothing if you send it to UINavigationController. You need to set toolbar items to that controller who is managed by a navigation controller. This line will make your buttons visible:
[self setToolbarItems:theItems animated:NO];