I started building a ship in single player and after placing a few blocks I used undo and crashed the game. Undo is a button I use a lot so it may have something to do with the latest release or the fleet release.
Starting the build I set all of the symmetry planes at the core.
Placed 50 blocks in the z axis then turned off the XY plane (blue)
place another fifty blocks in the aft portion of the z axis
This left a portion of the blocks outside of the symmetry planes and so i was not able to interact with them
this is probably where the nullpointer error gets thrown in undoing blocks that don't technically exist
Tried to use undo and crashed the game
I repeated these steps a few times and got the same results in both in single player and multiplayer