Not sure what this engine error is for. Here's a copy

Started getting this engine error when loading my save file. Not sure what it is.

develop-2924 (x64)
…/fixture_fabricator/fixture_fabricator_component.lua:155: attempt to index local ‘building’ (a nil value)
stack traceback:
[C]: ?
…/fixture_fabricator/fixture_fabricator_component.lua:155: in function ‘_are_dependencies_satisfied’
…/fixture_fabricator/fixture_fabricator_component.lua:202: in function ‘_start_project’
…/fixture_fabricator/fixture_fabricator_component.lua:41: in function <…/fixture_fabricator/fixture_fabricator_component.lua:39>

I remember that the way the Hearthlings build a structure is base on dependencies such as a wall being joined to a floor or a window to a wall. There were some tweaks that were done to improve the dependencies which fixed many unbuildable structures back in alpha 14. It you are able to upload the save game, some of the moderators could see what was causing the issue.