I want to base my design on an existing sample project. So I do File -> New from demo -> 4-Bedroom house

Then I try to save it but keep getting an error:


I'm able to save a completely empty project though.

Also, the same error happens if I copy everything from the 4-Bedroom house and paste into my empty project and try saving it.

By trial and error, I found that window is at least one problematic element. I can copy over walls and save, but as soon as I paste any window, I get that error. I seem to be able to add new windows from the tools though. It's sad that I cannot use the existing project and have to redraw it from scratch.