How to soft-reference a Blueprint A and load it in the Construction Script of a Blueprint B as child actor component?

Blueprint “A” has a simple InstancedStaticMesh Component in it. “A” is NOT placed in the World.
I want a second Blueprint “B” to use that InstancedStaticMesh from “A” as childActor Component.
This has to be done in the construction script of “B”, with soft-reference to “A”.
What’s wrong with the following setup? It seems that the cast is failing. Why is that?