Error launching turtlebot on custom world in Gazebo

Question

I created a world file by importing a mesh file (.stl).

I want to run the Turtlebot on my created world. I created a workspace and within the workspace I created a package. In the package I created the folders like worlds, launch, maps, etc..

Within the launch folder I created another folder includes. After that, I copied the kobuki.launch.xml file from the turtlebot_gazebo package to my package within the lunch/includes. I then copied turtlebot_world.launch file from turlebot_gazebo package to my package under launch folder. I modified the turtlebot_world.launch file so that it will open my created world with the turtlebot on it.

The code that I modified is as below

<launch>
  <arg name="world_file"  default="$(env TURTLEBOT_GAZEBO_WORLD_FILE)"/>

  <arg name="base"      value="$(optenv TURTLEBOT_BASE kobuki)"/> <!-- create, roomba -->
  <arg name="battery"   value="$(optenv TURTLEBOT_BATTERY /proc/acpi/battery/BAT0)"/>  <!-- /proc/acpi/battery/BAT0 --> 
  <arg name="gui" default="true"/>
  <arg name="stacks"    value="$(optenv TURTLEBOT_STACKS hexagons)"/>  <!-- circles, hexagons --> 
  <arg name="3d_sensor" value="$(optenv TURTLEBOT_3D_SENSOR kinect)"/>  <!-- kinect, asus_xtion_pro --> 

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="use_sim_time" value="true"/>
    <arg name="debug" value="false"/>
    <arg name="gui" value="$(arg gui)" />
   
     modified portion
    *<arg name="world_name" value="$(find gazebo_project)/worlds/my_mesh.world"/>*
  </include>
  
  <include file="$(find turtlebot_gazebo)/launch/includes/$(arg base).launch.xml">
    <arg name="base" value="$(arg base)"/>
    <arg name="stacks" value="$(arg stacks)"/>
    <arg name="3d_sensor" value="$(arg 3d_sensor)"/>
  </include>
  
  <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher">
    <param name="publish_frequency" type="double" value="30.0" />
  </node>
  
  <!-- Fake laser -->
  <node pkg="nodelet" type="nodelet" name="laserscan_nodelet_manager" args="manager"/>
  <node pkg="nodelet" type="nodelet" name="depthimage_to_laserscan"
        args="load depthimage_to_laserscan/DepthImageToLaserScanNodelet laserscan_nodelet_manager">
    <param name="scan_height" value="10"/>
    <param name="output_frame_id" value="/camera_depth_frame"/>
    <param name="range_min" value="0.45"/>
    <remap from="image" to="/camera/depth/image_raw"/>
    <remap from="scan" to="/scan"/>
  </node>
</launch>

However, when I launched the turtlebot_world.launch from my package it launches an empty world and a turtlebot on it. How can I solve this issue?