Have a custom
UIViewControllerTransitioningDelegate which, when its
presentationController(forPresented, presenting, source) method is called after a View Controller calls
presenting parameter is nil, which my method wasn’t expecting. Well, turns out, according to Swift docs…
This parameter may also be nil to indicate that the presenting view controller will be determined later.
Well, what a helpful parameter. So we need to handle this case, just in case the presenting view controller needs to be determined later for some reason.