RVIZ2 - PointCloud2 - No tf data

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Hello,

I have a very simple ROS2 node which is periodically publishing PointCloud2 messages. Now I want to show them in RVIZ2. After adding a PointCloud2 with Topic value "rtp" and changing the Fixed Frame value from "map" to "rt_frame" the points are displayed, but I still get the warning message "No tf data. Actual error: Frame [rt_frame] does not exist". Why? What causes this warning? How to fix it?

Furthermore, if I leave the Fixed Frame value at "map" I get the error message "Could not Transform from [rt_frame] to [map]".If I then try to manually add a TF object in RVIZ2 to define the transformation from "rt_frame" to "map" I cannot select any frames. Why can't I define a transformation in RVIZ2?

ROS2 node:

    #include <math.h>
    #include <stdio.h>
    #include <stdint.h>

    #include <chrono>

    #include <rclcpp/rclcpp.hpp>
    #include <rclcpp/time.hpp>

    #include "std_msgs/msg/header.hpp"

    #include "sensor_msgs/msg/point_cloud2.hpp"
    #include "sensor_msgs/point_cloud2_iterator.hpp"
    #include "sensor_msgs/msg/point_field.hpp"

    class RTN : public rclcpp::Node
    {

        public:

        RTN()
        : Node("RTN"), mCount(0)
        {
            mPublisher = this->create_publisher<sensor_msgs::msg::PointCloud2>("rtp", 1024);
            mTimer = this->create_wall_timer(std::chrono::milliseconds(500), std::bind(&RTN::timer_callback, this));
        }

    private:

        void timer_callback()
        {
            auto lPC = sensor_msgs::msg::PointCloud2();
            lPC.header.stamp = this->now();
            lPC.header.frame_id = "rt_frame";
            lPC.width=1;
            lPC.height=1;

            auto lPCM = sensor_msgs::PointCloud2Modifier(lPC);
            lPCM.setPointCloud2Fields(3, "x", 1, sensor_msgs::msg::PointField::FLOAT32, "y", 1, sensor_msgs::msg::PointField::FLOAT32, "z", 1, sensor_msgs::msg::PointField::FLOAT32);
            lPCM.resize(2);

            auto lXItr = sensor_msgs::PointCloud2Iterator<float>(lPC, "x");
            auto lYItr = sensor_msgs::PointCloud2Iterator<float>(lPC, "y");
            auto lZItr = sensor_msgs::PointCloud2Iterator<float>(lPC, "z");

            *lXItr = 0;
            *lYItr = 0;
            *lZItr = 0;
            ++lXItr; ++lYItr; ++lZItr;

            *lXItr = cosf(3.141f * 2 * 0.1f * (float)mCount);
            *lYItr = sinf(3.141f * 2 * 0.1f * (float)mCount);
            *lZItr = 0;
            ++lXItr; ++lYItr; ++lZItr;

            RCLCPP_INFO(this->get_logger(), "Publishing: '%i'", mCount++);
            mPublisher->publish(lPC);
        }

    private:

        rclcpp::TimerBase::SharedPtr mTimer;
        rclcpp::Publisher<sensor_msgs::msg::PointCloud2>::SharedPtr mPublisher;
        int mCount;

    };

    int main(int argc, char * argv[])
    {
        rclcpp::init(argc, argv);
        rclcpp::spin(std::make_shared<RTN>());
        rclcpp::shutdown();
        return 0;
    }