message: Fix a segfault when the object schema is unknown
diff: @@ -92,6 +92,9 @@ public:
  void add_instruction(Adapter::InstructionType type,
  nlohmann::json &&inst = {}, bool collapsible = false,
  util::Optional<std::string> object_type = util::none) {
  + if (!object_type && !m_selected_object_schema) {
  + return; // FIXME: support objects without schemas
  + }
  inst["type"] = Adapter::instruction_type_string(type);
  inst["object_type"] = object_type ? *object_type : m_selected_object_schema->name;
  m_json_instructions.push_back(std::move(inst));
  @@ -105,7 +108,7 @@ public:
  // collapse values if inserting/setting values for the last object
  if (m_last_is_collapsible) {
  nlohmann::json &last = m_json_instructions.back();
  - if (identity == last["identity"] && m_selected_object_schema->name == last["object_type"].get<std::string>()) {
  + if (identity == last["identity"] && m_selected_object_schema && m_selected_object_schema->name == last["object_type"].get<std::string>()) {
  last["values"][column] = value;
  return;
  }
  