Thanks everybody for your prompt responses. This is the response I got from Steve at Autoflight logic - just thought I would share it in case anyone else is having the same issue:
In some cases, there does appear to be a very infrequent issue of such "app crashing." We have inquired of DJI and are awaiting a response. In the mean time, have you tried deleting and reinstalling Autopilot? Do you have the GO app terminated and not running in the background? For restarting a mission from where you left off previously, you can move the mission start marker to any point on the flight path.
I have had GO running in the background - will also try the reinstall and let you know if it makes a difference...