All CraftBlockStates override the method to return the TileEntity.

Like in CraftChest:

 @Override
 public TileEntityChest getTileEntity() {
 return chest;
 }

or CraftBanner

 @Override
 public TileEntityBanner getTileEntity() {
 return banner;
 }

This Code is missing in CraftShulkerBox, which is why it returns null atm.