When opening the Zwift app skip the pairing screen when it first comes up. Just select Just Watch and pick your world and course. Once you enter the world click on menu and then the pairing screen and then wake up your devices and pair them.
Make sure you are not pedaling the Wattbike until after you are in the world.
Read this thread on the issue: (Fixed) Oct 16 update - crash iOS 13.1.2 - #27 by Stuart_Clements