nullptr when implement a ROS2 subscriber condition node

[SFhmichael]

Hi all, I am implement a BT condition node as a ROS2 subscriber, so that I can get some states from the topic and decide the condition node to return SUCCESS or FAILURE.
In BehaviorTree.ROS2/behaviortree_ros2/include/behaviortree_ros2/bt_topic_sub_node.hpp
https://github.com/BehaviorTree/BehaviorTree.ROS2/blob/ce923e19c12d28f4734a41b1442c123f2d4a41d2/behaviortree_ros2/include/behaviortree_ros2/bt_topic_sub_node.hpp#L97

  NodeStatus RosTopicSubNode<T>::tick()
{
.......
  auto status = CheckStatus (onTick(last_msg_));
  last_msg_ = nullptr;
.......
}

And the onTick function is implement in btcpp_ros2_samples/src/subscriber_test.cpp
https://github.com/BehaviorTree/BehaviorTree.ROS2/blob/ce923e19c12d28f4734a41b1442c123f2d4a41d2/btcpp_ros2_samples/src/subscriber_test.cpp#L20

NodeStatus onTick(const std::shared_ptr<std_msgs::msg::String>& last_msg) override
  {
    if(last_msg) // empty if no new message received, since the last tick 
    {
      RCLCPP_INFO(logger(), "[%s] new message: %s", name().c_str(), last_msg->data.c_str());
    }
    return NodeStatus::SUCCESS;
  }
};

What I can't understand is the reason that put last_msg_ = nullptr every tick, this means when the frequency of tick(ontick), is higher than the message published, and I want to use the content of topic to identify the condition to return SUCCESS or FAILURE. There will be a segmentation fault since the the next message doesn't come in and the content of pointer is null.
The following is my onTick function, and I will get segV because last_msg->data is empty.

NodeStatus onTick(const std::shared_ptr<std_msgs::msg::String>& last_msg) override
  {
    if(last_msg) // empty if no new message received, since the last tick
    {
      RCLCPP_INFO(logger(), "[%s] new message: %s", name().c_str(), last_msg->data.c_str());
    }
    if (last_msg->data == "work_done")
    {return NodeStatus::SUCCESS;}
    else
        return NodeStatus::FAILURE
  }
};