message: harden scene importer against missing lights
diff: @@ -55,7 +55,12 @@ OSPSG_INTERFACE void importScene(
  context->refreshScene(true);
  + if (lights == nullptr) {
  + std::cerr << "Scene file '" << fileName
  + << "' has no lights! Is this file correct?" << std::endl;
  + } else {
  context->frame->child("world").add(lights);
  + }
  CameraState cs = j["camera"];
  context->setCameraState(cs);
  