The game displaying blank doesn't sound like a good solution to me either? Or does this mean it's just not placing any custom structures on that territory?
This would only happen if a mod was bugged or doing something improper. As long as you're calling WL.StructureType.Custom you're fine, but if you bypass that function or manipulate the result of it then you'll get an invalid structure string. Prior to this update, a mod doing that would break the game for everyone that could see the invalid structure.