I get this error when I want to convert ducts and conduit joints to geometry:
What kind of elements do you have selected? One (or more) is returning a null. Can you identify that element and see if there’s any reason why that might be the case?
the elements are duct joints
Open your model detached, isolate a failing joint or two and any required elements to keep it existing (i.e. the ducts which it connects), and then delete everything else in the model including all views, materials, templates, filters, families, family types, sheets, and the like. Save that model and post it and this .dyn to the forum or a 3rd party service so others can replicate the issue.