Kamsta! That's an interesting crash, I will have a look at it today and see how I can stop that from happening. Thanks for the bug report.
I am interested in the changes you made for the assembler being a special electric furnace. I think you can have problems with it if the output is stuck or the input is set to something and wont let others be inserted.
I will prefer to make the droid assemblers just a big Chest which will spawn the next possible spawnable droid in the inventory. Then I can remove all the "dummy" items