Problem spawning in Gazebo a simple URDF model

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Hi there!

I'm using ROS2 Galactic and have a python package that simply spawn a robot. It works with a complex URDF model of a robot that have meshes and sensors, but what's driving me crazy is that it doesn't work with the following simple URDF model:

<?xml version="1.0"?>
<robot name="multipleshapes">
  <link name="base_link">
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </visual>
  </link>

  <link name="right_leg">
    <visual>
      <geometry>
        <box size="0.6 0.1 0.2"/>
      </geometry>
    </visual>
  </link>

  <joint name="base_to_right_leg" type="fixed">
    <parent link="base_link"/>
    <child link="right_leg"/>
  </joint>

</robot>

The URDF files are both in the same place. When I run the launch file to spawn the model it seems to work but the model doesn't appear in Gazebo. I don't get errors.

ros@ubuntu:~/ros2_ws$ ros2 launch mars_robot mars_robot.launch.py
[INFO] [launch]: All log files can be found below /home/ros/.ros/log/2022-05-17-18-57-27-755134-ubuntu-24691
[INFO] [launch]: Default logging verbosity is set to INFO
[INFO] [gzserver-1]: process started with pid [24693]
[INFO] [gzclient   -2]: process started with pid [24696]
[INFO] [robot_state_publisher-3]: process started with pid [24699]
[INFO] [spawn_entity.py-4]: process started with pid [24701]
[robot_state_publisher-3] Link right_leg had 0 children
[robot_state_publisher-3] [INFO] [1652806649.104605067] [robot_state_publisher]: got segment base_link
[robot_state_publisher-3] [INFO] [1652806649.106763959] [robot_state_publisher]: got segment right_leg
[spawn_entity.py-4] [INFO] [1652806650.703018213] [spawn_entity]: Spawn Entity started
[spawn_entity.py-4] [INFO] [1652806650.707231388] [spawn_entity]: Loading entity published on topic robot_description
[spawn_entity.py-4] [INFO] [1652806650.711765040] [spawn_entity]: Waiting for entity xml on robot_description
[spawn_entity.py-4] [INFO] [1652806650.713877199] [spawn_entity]: Waiting for service /spawn_entity, timeout = 5
[spawn_entity.py-4] [INFO] [1652806650.717389319] [spawn_entity]: Waiting for service /spawn_entity
[spawn_entity.py-4] [INFO] [1652806653.745771815] [spawn_entity]: Calling service /spawn_entity
[spawn_entity.py-4] [INFO] [1652806653.975522539] [spawn_entity]: Spawn status: SpawnEntity: Successfully spawned entity [mars_robot]
[spawn_entity.py-4] /opt/ros/galactic/lib/python3.8/site-packages/rclpy/qos.py:307: UserWarning: DurabilityPolicy.RMW_QOS_POLICY_DURABILITY_TRANSIENT_LOCAL is deprecated. Use DurabilityPolicy.TRANSIENT_LOCAL instead.
[spawn_entity.py-4]   warnings.warn(
[INFO] [spawn_entity.py-4]: process has finished cleanly [pid 24701]

The code of the launch file is the following:

import os
import xacro

from ament_index_python.packages import get_package_share_directory
from launch import LaunchDescription
from launch.actions import IncludeLaunchDescription 
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch_ros.actions import Node
from launch.actions import SetEnvironmentVariable
from launch.substitutions import EnvironmentVariable

def generate_launch_description():

    pkg_install_path = get_package_share_directory('mars_robot')

    if 'GAZEBO_MODEL_PATH' in os.environ:
        model_path =  os.environ['GAZEBO_MODEL_PATH'] + ':' + pkg_install_path
    else:
        model_path =  pkg_install_path


    xacro_robot_pkg_path = os.path.join(
        get_package_share_directory('mars_robot'))

    xacro_file = os.path.join(xacro_robot_pkg_path,
                              'urdf',
                              'prueba.urdf')

    doc = xacro.parse(open(xacro_file))
    xacro.process_doc(doc)

    node_robot_state_publisher = Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        output='screen',
        parameters=[{'robot_description': doc.toxml()}]
    )

    spawn_entity = Node(package='gazebo_ros', executable='spawn_entity.py',
                        arguments=['-topic', 'robot_description',
                                   '-entity', 'mars_robot'],
                        output='screen')

    gazebo = IncludeLaunchDescription(
                PythonLaunchDescriptionSource([os.path.join(
                    get_package_share_directory('gazebo_ros'), 'launch', 'gazebo.launch.py')]),
             )

    return LaunchDescription([
        gazebo,
        node_robot_state_publisher,
        spawn_entity,
    ])

I've been trying to solve the problem for several days but I can't find the solution. Can ... (more)