After restarting Xcode, cleaning the build and a number of the other suggestions on SO about this problem. What actually fixed it for me was a hard reboot of the device. (I wasted over an hour on this)
I suppose it just goes to show that the old saying have you turned it off and on again is still true for the latest gadgets!
How to reboot iPhone XS or iPhone XS Max
- Quickly press and release the Volume Up button
- Quickly press and release the Volume Down button
- Press and hold the Side Button until the Apple logo appears