message: Fix crash when attempting to load older .sg file with lights
diff: @@ -37,10 +37,12 @@ bool LightsManager::addLight(NodePtr light)
  // When adding HDRI or sunSky, set background color to black.
  // It's otherwise confusing. The user can still adjust it afterward.
  if (light->subType() == "hdri" || light->subType() == "sunSky") {
  + if (!parents().empty()) {
  auto &frame = parents().front();
  auto &renderer = frame->childAs<sg::Renderer>("renderer");
  renderer["backgroundColor"] = rgba(vec3f(0.f), 1.f); // black, opaque
  }
  + }
  lightNames.push_back(light->name());
  add(light);
  