Gazebo hangs with black screen

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Hi, I designed a robot in URDF and opened it in Gazebo. The model seems to work perfectly. When I tried to add "libgazebo_ros_control.so", the screen goes dark and nothing works. Here is my code:

    <?xml version="1.0" encoding="utf-8"?>
<robot name="toycar">
  <link name="base_link">
    <inertial>
      <origin xyz="-0.0499255895387698 -0.000237637381050919 0.0440894807015004" rpy="0 0 0"/>t
      <mass value="1.39579817055882"/>
      <inertia ixx="0.00548170445188548" ixy="6.54622543058193E-05" ixz="0.000189403535314588" iyy="0.00659169387832073" iyz="-3.53806937867786E-05" izz="0.00798757046823767"/>
    </inertial>
    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://toycar/meshes/base_link.STL"/>
      </geometry>
      <material name="">
        <color rgba="0 0 0 0 "/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://toycar/meshes/base_link.STL"/>
      </geometry>
    </collision>
  </link>
  <link name="R_link">
    <inertial>
      <origin xyz="-1.85861074863958E-08 -1.31927272231658E-09 0.0181270123819038" rpy="0 0 0"/>
      <mass value="0.166945648008928"/>
      <inertia ixx="6.72964134165818E-05" ixy="5.95215618820829E-12" ixz="3.19966218203191E-12" iyy="6.72926792753019E-05" iyz="2.24048319173707E-13" izz="0.00011320641158774"/>
    </inertial>
    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://toycar/meshes/R_link.STL"/>
      </geometry>
      <material name="">
        <color rgba="0.792156862745098 0.819607843137255 0.933333333333333 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://toycar/meshes/R_link.STL"/>
      </geometry>
    </collision>
  </link>
  <joint name="r_joint" type="continuous">
    <origin xyz="0 0.0663626778251484 0" rpy="1.5707963267949 -0.0503110573119223 3.14159265358979"/>
    <parent link="base_link"/>
    <child link="R_link"/>
    <axis xyz="0 0 1"/>
  </joint>
  <link name="L_link">
    <inertial>
      <origin xyz="-1.8586106820262E-08 -1.31927103269591E-09 0.0181270123819035" rpy="0 0 0"/>
      <mass value="0.16694564800893"/>
      <inertia ixx="6.72964134165798E-05" ixy="5.95215545891293E-12" ixz="3.19966216708776E-12" iyy="6.72926792753048E-05" iyz="2.24048103784762E-13" izz="0.000113206411587742"/>
    </inertial>
    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://toycar/meshes/L_link.STL"/>
      </geometry>
      <material name="">
        <color rgba="0.792156862745098 0.819607843137255 0.933333333333333 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://toycar/meshes/L_link.STL"/>
      </geometry>
    </collision>
  </link>
  <joint name="l_link" type="continuous">
    <origin xyz="0 -0.070423380902808 0" rpy="1.5707963267949 -0.242125828866567 0"/>
    <parent link="base_link"/>
    <child link="L_link"/>
    <axis xyz="0 0 1"/>
  </joint>
  <link name="R_Caster">
    <inertial>
      <origin xyz="-0.00166674599734672 -0.0485894807015005 0.0960273248225965" rpy="0 0 0"/>
      <mass value="1.39579817055882"/>
      <inertia ixx="0.00631959121652698" ixy="7.39735297430925E-05" ixz="0.000481950750263459" iyy="0.00798757046823767" iyz="-0.000177914051102901" izz="0.00575380711367924"/>
    </inertial>
    <visual>
      <origin xyz="0 0 0" rpy="0 0 ...