message: refactor: avoid exception by using json operator[]
diff: @@ -252,7 +252,7 @@ namespace Nextclade {
  bool isReferenceNode() const {
  ensureIsObject();
  - const auto nodeType = j.value(json_pointer{"/node_attrs/Node Type/value"}, json());
  + const auto& nodeType = j[json_pointer{"/node_attrs/Node Type/value"}];
  return nodeType.is_string() && (nodeType.get<std::string>() == "Reference");
  }
  