课程提供是实例代码，启动gazibo是报错

[ERROR] [1596790820.242599432]: Failed to build tree: parent link [base_link] of joint [lidar_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 [lidar_joint] from your urdf file, or add “<link name=”base_link” />” to your urdf file.
[robot_state_publisher-5] process has died [pid 73099, exit code 1, cmd /opt/ros/noetic/lib/robot_state_publisher/robot_state_publisher __name:=robot_state_publisher __log:=/home/zxb/.ros/log/6b596a3e-d88c-11ea-b9ca-4bd372888d5d/robot_state_publisher-5.log].
log file: /home/zxb/.ros/log/6b596a3e-d88c-11ea-b9ca-4bd372888d5d/robot_state_publisher-5*.log

小车在gazibo中不显示