I was playing the testing branch of the Spaced Out! DLC, and messing around in sandbox and debug mode. (I also had Insta-build on) I was testing the new radbolt rocket engine, and everything seemed to be going well. I filled up the rocket, put on a cartography module as well as a battery. I put on a spacefarer module but couldn't fit the nosecone. so I chose the option to replace it with a solo spacefarer nosecone, but the game crashed as soon as I did it. Still not sure what's up.
No mods were in use at the time of the crash
EDIT: I did the same thing on a brand new save and the same thing happened.
create radbolt engine (empty OR full); add a Spacefarer module (big one); choose replace module and choose solo spacefarer nosecone