That probably is the problem, yes.
You may as well change them to straight object references as they will already be loaded into memory if the volume is placed in your level and I can’t imagine you’ll see any benefit by using a soft reference.
Otherwise, if for some reason you are set on using the soft reference, before you can trigger that event, you will need to Resolve that soft reference by using the Resolve Soft Reference node if it’s already loaded into memory or Casting to the volume.