You’ll probably want to add some more debug functionality like printing out how many actors / components it finds. What I think is happening is that it isn’t finding any HISM actors. That ForestUnderstory actor is just a plain old actor that happens to have a HISM component. So next thing to change would be to filter by components instead of actors (actually, I can’t see if that’s what you’re doing.) Or filter by name and then get the component to make sure you’re getting access to it properly.
The other thing is that HISMs might not get updated inside the Dataprep editor, so you’ll have to commit the import to the project to see if it worked.