message: only decrease ref count of object parameters if the object exists
diff: @@ -31,8 +31,10 @@ namespace ospray {
  params_end(),
  [&](std::shared_ptr<Param> &p) {
  auto &param = *p;
  - if (param.data.is<OSP_PTR>())
  - param.data.get<OSP_PTR>()->refDec();
  + if (param.data.is<OSP_PTR>()) {
  + auto *obj = param.data.get<OSP_PTR>();
  + if (obj != nullptr) obj->refDec();
  + }
  });
  }
  