老师，b站视频课6.2.2，这里我按照视频里的代码写的，但是在执行的时候报错了

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下面是我的代码
import launch
import launch_ros
from ament_index_python.packages import get_package_share_directory
import os
import launch_ros.parameter_descriptions

def generate_launch_description():
urdf_package_path=get_package_share_directory('fishbot_description')
default_urdf_path=os.path.join(urdf_package_path,'urdf','first_robot.urdf')

action_declare_arg_mode_path=launch.actions.DeclareLaunchArgument(
    name='model',default_value=str(default_urdf_path),description='加载的模型文件路径'
)

substitutions_result=launch.substitutions.Command(['cat ',launch.substitutions.launch_configuration('model')])
robot_description_value=launch_ros.parameter_descriptions.ParameterValue(substitutions_result,value_type=str)



action_robot_state_publisher=launch_ros.actions.Node(
    package='robot_state_publisher',
    executable='robot_state_publisher',
    parameters=[{'robot_description':robot_description_value}]
)


action_joint_state_publisher=launch_ros.actions.Node(
    package='joint_state_publisher',
    executable='joint_state_publisher',
)

action_rviz_node=launch_ros.actions.Node(
    package='rviz2',
    executable='rviz2',
)
return launch.launch_description([
    action_declare_arg_mode_path,
    action_robot_state_publisher,
    action_joint_state_publisher,
    action_rviz_node,
])