No parameterless constructor defined for this object?

Hey all

I am working on a GH sketch (See attached file below) which is a parametric system for making vehicle frames. Im trying to create a UI for this using human UI.

I am getting the following error on a UI container object. Solution exception : no parameterless constructor defined for this object. Have no Idea what this is and why it should be showing. I’ve done everything right.

PL help out, its kinda urgent and Im new to this!!
Parametric BUCK _HUI_First test.gh (57.4 KB)