[2871] Game prohibits me from finishing a slab addition to a building built by the editor

[Yosmo78]

Summary:
Game prohibits me from finishing a slab addition to a building built by the editor
Steps to reproduce:

1. open up Dropbox - 1457219143970.zip - Simplify your life
2. immediately open up build editor and try to add blocks between white blocks
3. produces error
   (I originally built a regular building then tried to create something out of slab that hung off then down from it an it become really glitch)
   do this:
   
   
   Expected Results:
   puts a block there
   Actual Results:
   bugs out
   Notes:
   develop-2871 (x64)
   stonehearth/lib/build_util.lua:332: assertion failed!
   stack traceback:
   radiant/modules/common.lua:223: in function ‘report_traceback’
   radiant/modules/common.lua:234: in function <radiant/modules/common.lua:228>
   [C]: in function ‘assert’
   stonehearth/lib/build_util.lua:332: in function ‘can_start_blueprint’
   …earth/components/fabricator/fabricator_component.lua:727: in function ‘_start_project’
   …earth/components/fabricator/fabricator_component.lua:530: in function ‘_updates_state’
   …earth/components/fabricator/fabricator_component.lua:519: in function ‘obj’
   radiant/modules/events.lua:73: in function ‘instance’
   radiant/modules/events.lua:206: in function <radiant/modules/events.lua:200>
   [C]: in function ‘xpcall’
   radiant/modules/common.lua:243: in function ‘xpcall’
   radiant/modules/events.lua:200: in function ‘trigger’
   radiant/modules/events.lua:269: in function ‘_trigger_gameloop’
   radiant/modules/events.lua:320: in function ‘_update’
   radiant/server.lua:62: in function <radiant/server.lua:58>
   Attachments:

Version Number and Mods in use:
alpha 15 2871
System Information: