Default ships are fine it starts as soon as i add my own ship or after i have altered a default ship, once the change is made i can no longer edit/add or copy without that error.
Tested every way clean install with and without mods.
Which version of DW Universe are you using? (the version number is at the bottom-left of the startup screen) There was a bug like this in the ship design editor that was fixed in the 1.9.5.1 update.