[gazebo-2] process has died [pid 4088, exit code 139, cmd /opt/ros/melodic/lib/gazebo_ros/gzserver -e ode /home/zhang/clean06a_ws/src/clean_ct06a/worlds/CleanCT06A.world

edit

Why does it happen sometimes when i launch the file as shown after

<launch>

  <arg name="urdf_file" default="$(find xacro)/xacro.py '$(find clean_ct06a)/urdf/mbot/mbot_gazebo.xacro'" />
  <param name="robot_description" command="$(arg urdf_file)" />

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="world_name" value="$(find clean_ct06a)/worlds/CleanCT06A.world"/>
    <arg name="paused" value="false"/>
    <arg name="use_sim_time" value="true"/>
    <arg name="gui" value="true"/>
    <arg name="headless" value="false"/>
    <arg name="debug" value="false"/>
  </include>

  <node pkg="gazebo_ros" type="spawn_model" name="spawn_urdf" args="-urdf  -model wheelcar -param robot_description" />
</launch>