NullReferenceException in AssignmentGroupController.OnSpawn

I have a similar crash log, where there is a error caused by the rocket. I have done some testing to what this might be, and found out that as soon as I assign a crew of duplicants to the module (click on the "crew" button and tick the dupes I want to send on a mission), then wait until an autosave occurs (or even save manually) and for the final step, I open the pause menu, and load the same savefile. Same behavior occurs when I freshly start the game and load the save via the start menu.

I have to admit that I am experimenting with "out-of-bound" rocket designs, meaning that I have melted the walls and expanded the available rocket interior space. But I didn't experience any bugs or crashes while working this way, it seems to behave orderly.

I did some further testing. The same design, with the same parameters, on a freshly created map didn't crash the game after some tweaking. I found out that the lowest tiles should not contain pipes/cables/conveyors that connect "upwards" because they get severed during a reload. I once crashed the game when trying to connect a gas pipe from the lowest tile to the one below. But this was not a reload crash.

Even after assigning duplicants to crew, and removing the assignment, saving, reloading no crashes were observed.

On the old map where the savefile corruption would happen consistently, assigning duplicants to be crew of another rocket had the same effect. Creating a new rocket, and assigning the duplicants produced a corrupted save file too.