How can I use one of the UIBarButtonSystemItems if my UIBarButtonItem is created via splitViewController:willHideViewController:?
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I see in the documentation that if I create a UIBarButtonItem with `initWithBarButtonSystemItem:target:action:`, I can choose one of the UIBarButtonSystemItems.

However, my UIBarButtonItem is being created for me with `splitViewController:willHideViewController:withBarButtonItem:forPopoverController:`. Obviously, inside that method I can set my UIBarButtonItem's title or style, but I can't seem to change it to one of the UIBarButtonSystemItems. Is it possible to set it to one of those inside that method?