I haven't tried reproducing at this time, and I have no real idea how to trigger the second one - it felt quite random given I had been doing calibrations before and later without error. I did have use dark frame switched on and I might have adjusted the exposure time and/or pulse (but I can't be sure).
The first one though, I think I should be able to reproduce next time I go out, as I could see a likely cause:
Calibration panel
1. Capture
2. Select Star
3. Do Calibration
Switch to Guiding Panel
1. Click sub frame
2. Hit capture
3. Forget/note that the star doesn't have the green box around it any more and therefore isn't selected
4. Hit autoguide and crash
Well, it's not suppose to crash even if no star is selected after subframe, but I make it move the guide square now to the subframe coords automatically, not sure if this will make any difference.