Alan J. Steinberg is out in left field with this supposition. Trump will not be resigning in 2019. Even if Trump for some reason is not able to finish his term, Pence would be a suitable replacement. As far as Chris Christy is concerned, he can forget ever being elected President. Republican voters just do not vote that way. If he were the candidate, it would insure a democratic victory.
Chris Christie has a much better chance of making commercials for Nutrisystem than winning nomination.