Creating a spline from a mesh

I’m trying to get nulls cloned to a cube so that I can create a spline connecting the points of the cube. The purpose is to be able to then use effectors to push these nulls around.
However if I clone nulls to a Shape 3d with a “Clone to Mesh”, then pipe the clone to mesh in the Spline Nodes port of the Spline operator, I don’t get anything.

What the right approach here?