Crash when attempting to stack an item with no subgroup

[Mernom]

Error log:

Failed to load mods: __deadlock-beltboxes-loaders__/prototypes/create_stack.lua:19: attempt to index field '?' (a nil value)
stack traceback:
	__deadlock-beltboxes-loaders__/prototypes/create_stack.lua:19: in function 'get_group'
	__deadlock-beltboxes-loaders__/prototypes/create_stack.lua:79: in function 'create_stacked_item'
	__deadlock-beltboxes-loaders__/prototypes/public.lua:194: in function 'add_stack'
	__personal__/data-final-fixes.lua:104: in main chunk

Code to reproduce:

if deadlock_stacking then
  local new_item = table.deepcopy (data.raw.item.wood)
  new_item.name = "test-item"
  new_item.subgroup = nil
  data:extend { new_item }
  deadlock.add_stack ( "test-item" )
end

Suggestion: add a fallback to the ... [data.raw[item_type][item].subgroup].group ... part.