i keep having this error when i try to launch my urdf model

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REQUIRED process [gazebo-4] has died! process has died [pid 9348, exit code 255, cmd /opt/ros/kinetic/lib/gazebo_ros/gzserver /home/mohamed/catkin_ws/src/m3a_pro/ctcv_gazebo/worlds/ctcv.world __name:=gazebo __log:=/home/mohamed/.ros/log/1e2530e4-ec71-11e8-89ca-a088699ada62/gazebo-4.log]. log file: /home/mohamed/.ros/log/1e2530e4-ec71-11e8-89ca-a088699ada62/gazebo-4*.log Initiating shutdown!

my URDF FILE <robot name="robotcraft2018">

<material name="black"> <color rgba="0 0 0 1"/> </material> <material name="white"> <color rgba="1 1 1 1"/> </material> <material name="red"> <color rgba="0.8 0 0 1"/> </material> <material name="green"> <color rgba="0 1 0 1"/> </material> <material name="purple"> <color rgba="0.5 0 0.5 1"/> </material> <material name="transparent"> <color rgba="0.6 0.6 0.6 0.35"/> </material>

<link name="base_link"> <visual> <origin rpy="0 0 3.14159265359" xyz="0.03 -0.005 -0.09"/> <geometry> <mesh filename="package://ctcv_gazebo/mesh/Black.stl" scale="0.001 0.001 0.001"/> </geometry> <material name="black"/> </visual>

<collision> <origin rpy="0 0 3.14159265359" xyz="0.03 -0.005 -0.09"/> <geometry> <mesh filename="package://ctcv_gazebo/mesh/Black.stl" scale="0.001 0.001 0.001"/> </geometry> </collision>

<inertial>
    <origin rpy="0 0 3.14159265359" xyz="0.03 -0.005 -0.09"/>
    <mass value="350"/>
    <inertia ixx="2.0" ixy="0.0" ixz="0.0" iyy="2.0" iyz="0.0" izz="2.0"/>
</inertial>

</link>

<gazebo reference="base_link"> <material>Gazebo/Black</material> </gazebo>

<link name="bottom_base"> <visual> <geometry> <mesh filename="package://ctcv_gazebo/mesh/White.stl" scale="0.001 0.001 0.001"/> </geometry> <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.11"/> <material name="white"/> </visual> <collision> <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.11"/> <geometry> <mesh filename="package://ctcv_gazebo/mesh/White.stl" scale="0.001 0.001 0.001"/> </geometry> </collision>

<inertial>
    <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.11"/> 
    <mass value="350"/>
    <inertia ixx="2.0" ixy="0.0" ixz="0.0" iyy="2.0" iyz="0.0" izz="2.0"/>
</inertial>

</link>

<gazebo reference="bottom_base"> <material>Gazebo/White</material> </gazebo>

<link name="acrylic_base"> <visual> <geometry> <mesh filename="package://ctcv_gazebo/mesh/Transparent.stl" scale="0.001 0.001 0.001"/> </geometry> <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.145"/> <material name="transparent"/> </visual> <collision> <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.145"/> <geometry> <mesh filename="package://ctcv_gazebo/mesh/Transparent.stl" scale="0.001 0.001 0.001"/> </geometry> </collision>

<inertial>
    <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.145"/>
    <mass value="350"/>
    <inertia ixx="2.0" ixy="0.0" ixz="0.0" iyy="2.0" iyz="0.0" izz="2.0"/>
</inertial>

</link>

<gazebo reference="acrylic_base"> <material>Gazebo/GreyTransparent</material> </gazebo>

<link name="arduino"> <visual> <geometry> <mesh filename="package://ctcv_gazebo/mesh/Green.stl" scale="0.001 0.001 0.001"/> </geometry> <origin rpy="0 0 3.14159265359" xyz="0 -0.005 -0.17"/> <material name="green"/> </visual> <collision> <origin rpy="0 0 ... (more)