Failed to build tree: child link [base_link] of joint [world_joint] not found

Hi all hope you all are doing great.

I have this error:

not single time but multiple times. also some more:

[robot_state_publisher-4] process has died [pid 10139, exit code 255, cmd /opt/ros/kinetic/lib/robot_state_publisher/robot_state_publisher name:=robot_state_publisher log:=/home/ros/.ros/log/eed9c1bc-a5ca-11ec-a955-d80f997b25c9/robot_state_publisher-4.log]. log file: /home/ros/.ros/log/eed9c1bc-a5ca-11ec-a955-d80f997b25c9/robot_state_publisher-4.log [ur5_mp-14] process has died [pid 10216, exit code 1, cmd /home/ros/ur_ws/src/ur5_ROS-Gazebo/ur5_mp.py /follow_joint_trajectory:=/arm_controller/follow_joint_trajectory __name:=ur5_mp __log:=/home/ros/.ros/log/eed9c1bc-a5ca-11ec-a955-d80f997b25c9/ur5_mp-14.log]. log file: /home/ros/.ros/log/eed9c1bc-a5ca-11ec-a955-d80f997b25c9/ur5_mp-14.log

I am using this git repository:

https://github.com/lihuang3/ur5_ROS-G...

I tried following this tutorial , I had an issue of ur_hardware_interface which this git addresses and I was able to solve it. the error new I am facing (i.e. failed to build tree), i have searched it and found solutions on it but not specific to my case and theirs dont apply to my case.

here is my initialize.launch :

<?xml version="1.0"?>

and here is my ur5_joint_limited_robot.urdf.xacro: (modified according to the git)

<robot <a="" href="http://xmlns:xacro="http://wiki.ros.org/xacro">xmlns:xacro="http://wiki.ros.org/xacro" name="ur5" ><="" p="">

Please advise with this issue, also Please answer these questions: 1. why do a failed to build tree error occures? 2. how doee one ensure a urdf is correctly made? 3. Any tips on how to remove the warning: inconsistent namespace redefinitions for xmlns:xacro

I know we can run things with warning so you may not answer the last question.

lihuang3 / ur5_ROS-Gazebo

Failed to build tree: child link [base_link] of joint [world_joint] not found #42