Missing link in manipulator urdf /xacro and visualizing in rviz

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Hello everyone,

I am trying to write a launch file in order to visualize a manipulator in Rviz. The manipulator can be found here:

https://github.com/ipa320/schunk_modu...

I am using the xacro file lwa.urdf.xacro to build my robot.

I created another xacro file where I applied the macro in the previous urdf.xacro file. It looks like this:

<?xml version="1.0" ?>
<robot name="schunk_lwa" xmlns:xacro="http://ros.org/wiki/xacro">
  <xacro:include filename="$(find schunk_description)/urdf/lwa/lwa.urdf.xacro"/>
  <xacro:schunk_lwa parent="base_link" name="Schunk_lwa"> 
    <origin xyz="0 0 0 "/>
   </xacro:schunk_lwa>
</robot>

My launch file looks like this :

    <launch>
        <arg name="model" />

        <!-- Parsing xacro and setting robot_description parameter -->
        <param name="robot_description" command="$(find xacro)/xacro $(find schunk_description)/urd/lwa/lwa.xacro"/>

        <!-- Setting gui parameter to true for display joint slider -->
        <param name="use_gui" value="true"/>

        <!-- Starting Joint state publisher node which will publish the joint values -->
        <node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" />

        <!-- Starting robot state publish which will publish tf -->
        <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher" />

        <!-- Launch visualization in rviz -->
        <node name="rviz" pkg="rviz" type="rviz" args="-d $(find mastering_ros_robot_description_pkg)/urdf.rviz" required="true" />
 </launch>

However, when I run the launchfile, the error occurs:

[ERROR] [1585922552.126688366]: Failed to build tree: parent link [base_link] of joint [Schunk_lwa_base_joint] not found.  This is not valid according to the URDF spec. Every link you refer to from a joint needs to be explicitly defined in the robot description. To fix this problem you can either remove this joint [Schunk_lwa_base_joint] from your urdf file, or add "<link name="base_link" />" to your urdf file.
[robot_state_publisher-2] process has died [pid 12234, exit code 1, cmd /opt/ros/melodic/lib/robot_state_publisher/robot_state_publisher __name:=robot_state_publisher __log:=/home/phan/.ros/log/a26135c2-75a2-11ea-a8dd-a0afbd22d805/robot_state_publisher-2.log].
log file: /home/phan/.ros/log/a26135c2-75a2-11ea-a8dd-a0afbd22d805/robot_state_publisher-2*.log
[WARN] [1585922553.516583, 5790.022000]: The 'use_gui' parameter was specified, which is deprecated.  We'll attempt to find and run the GUI, but if this fails you should install the 'joint_state_publisher_gui' package instead and run that.  This backwards compatibility option will be removed in Noetic.
[ERROR] [1585922553.542646165, 5790.047000000]: Failed to build tree: parent link [base_link] of joint [Schunk_lwa_base_joint] not found.  This is not valid according to the URDF spec. Every link you refer to from a joint needs to be explicitly defined in the robot description. To fix this problem you can either remove this joint [Schunk_lwa_base_joint] from your urdf file, or add "<link name="base_link" />" to your urdf file.

From the error I assume that the base link is missing. I therefore added to the above urdf.xacro file the following pseudo base link:

<link name="${parent}">
</link>

Using this, it works.

However, my question is, why is in the original model no base link defined? I am afraid I am missing something ... (more)