launch file doesn't launch anything

edit

hello I've decided to create 2 robot and launch them in gazebo.when I run the launch file, it doesn't show any errors and also doesn't show anything on the screen.gazebo is preparing world.I use ubuntu 16 and ros kinetic

<?xml version="1.0"?>
<launch>

  <arg name="first_tb3"  default="tb3_0"/>
  <arg name="second_tb3" default="tb3_1"/>
 <arg name="world_name" default="worlds/empty.world"/>  

  <arg name="first_tb3_x_pos" default="0.0"/>
  <arg name="first_tb3_y_pos" default="0.0"/>
  <arg name="first_tb3_z_pos" default="0.0"/>
  <arg name="first_tb3_yaw"   default="0.0"/>

  <arg name="second_tb3_x_pos" default=" 0.0"/>
  <arg name="second_tb3_y_pos" default="-1.0"/>
  <arg name="second_tb3_z_pos" default=" 0.0"/>
  <arg name="second_tb3_yaw"   default=" 0.0"/>


<include file="$(find gazebo_ros)/launch/empty_world.launch">
<arg name="world_name" value="$(arg world_name)"/> 
<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>  

 <group ns = "$(arg first_tb3)">
    <param name="robot_description" command="$(find xacro)/xacro --inorder $(find husky_description)/urdf/husky.urdf.xacro" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" output="screen">
      <param name="publish_frequency" type="double" value="50.0" />
      <param name="tf_prefix" value="$(arg first_tb3)" />
    </node>

 <node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model $(arg first_tb3) -x $(arg first_tb3_x_pos) -y $(arg first_tb3_y_pos) -z $(arg first_tb3_z_pos) -Y $(arg first_tb3_yaw) -param robot_description" />
  </group>

  <group ns = "$(arg second_tb3)">
    <param name="robot_description" command="$(find xacro)/xacro --inorder $(find husky_description)/urdf/husky.urdf.xacro" />

    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" output="screen">
      <param name="publish_frequency" type="double" value="50.0" />
      <param name="tf_prefix" value="$(arg second_tb3)" />
    </node>

 <node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model" args="-urdf -model $(arg second_tb3) -x $(arg second_tb3_x_pos) -y $(arg second_tb3_y_pos) -z $(arg second_tb3_z_pos) -Y $(arg second_tb3_yaw) -param robot_description" />
  </group>

</launch>