Fixed several bugs.

● Made another attempt to fix the bug where terrain chunks would disappear after digging the ground surface. This issue involves the voxel system and is extremely complex. If the bug still occurs after this update, please report it on Discord or the Steam discussion forums. Thank you!

● Fixed a bug where writing non-player-built scene models into the building save file would prevent the game from exiting.

● Fixed a bug where after loading a save, repairing a damaged building structure would cause the object to disappear.

● Fixed a null reference bug in the mesh batching system.

● Fixed a bug where placing a "Door" on a vehicle would cause no visual change when opening or closing the door.