Here is a debug.err file for attempting to do this crash. (I figured it out and can reproduce it reliably now)
Here is how to make the designer crash.
(I use the Irridium)
Go to the designer.
Use a tiny hull (probably doesn't matter which one you use)
Add a piece (I used Irridium model 30 attached to the side of the cube)
Add another piece to the piece you placed (I added Irridium model 09)
So it looks like this picture.
Do symmetry on piece Irridium09 (the piece you added last)
Do symmetry on the former piece (Irridium Model 30)
then remove symmetry from that piece.
The problem now is that the game think that the piece Irridium Model 09 is still in symmetry mode, but it's not. So deselecting symmetry on that piece will crash the game.
I apologize if this doesn't make sense. If you need more guidance, let me know and I'll try to set up a youtube video for you.