Fixed several bugs caused by null reference objects.

● Fixed a null reference bug when loading surrounding buildings.

● Fixed a null reference bug that occurred after building a "Door".

● Fixed a null reference bug with vehicle tires.

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

● Fixed a null reference bug when aligning objects in the building system.

● Fixed a null reference bug when detecting adjacent objects in the building system.

● Fixed a rendering misalignment bug of terrain trees.