I am new to Ubuntu,ROS and Gazebo. I have created a simple robot and trying to add laser scanner in it using this tutorial http://gazebosim.org/tutorials?tut=add_laser but when I use this code and try to drag robot on plan it start loading and don't give any error message and it also don't show database models "connecting to database models". What i am missing please help me. the code of robot model is this

<?xml version="1.0"?>
<sdf version="1.4">
  <model name="my_robot">
    <static>false</static>
    <link name="chassis">
      <pose>0 0 .1 0 0 0</pose>
      <collision name="collision">
        <geometry>
          <box>
            <size>.4 .2 .1</size>
          </box>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <box>
            <size>.4 .2 .1</size>
          </box>
        </geometry>
      </visual>
      <collision name="caster_collision">
        <pose>-0.15 0 -0.05 0 0 0</pose>
        <geometry>
          <sphere>
            <radius>.05</radius>
          </sphere>
        </geometry>
        <surface>
          <friction>
            <ode>
              <mu>0</mu>
              <mu2>0</mu2>
              <slip1>1.0</slip1>
              <slip2>1.0</slip2>
            </ode>
          </friction>
        </surface>
      </collision>
      <visual name="caster_visual">
        <pose>-0.15 0 -0.05 0 0 0</pose>
        <geometry>
          <sphere>
            <radius>.05</radius>
          </sphere>
        </geometry>
      </visual>
    </link>
    <link name="left_wheel">
      <pose>0.1 0.13 0.1 0 1.5707 1.5707</pose>
      <collision name="collision">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </visual>
    </link>
    <link name="right_wheel">
      <pose>0.1 -0.13 0.1 0 1.5707 1.5707</pose>
      <collision name="collision">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <cylinder>
            <radius>.1</radius>
            <length>.05</length>
          </cylinder>
        </geometry>
      </visual>
    </link>
    <joint type="revolute" name="left_wheel_hinge">
      <pose>0 0 -0.03 0 0 0</pose>
      <child>left_wheel</child>
      <parent>chassis</parent>
      <axis>
        <xyz>0 1 0</xyz>
      </axis>
    </joint>
    <joint type="revolute" name="right_wheel_hinge">
      <pose>0 0 0.03 0 0 0</pose>
      <child>right_wheel</child>
      <parent>chassis</parent>
      <axis>
        <xyz>0 1 0</xyz>
      </axis>
    </joint>
    <include>
      <uri>model://hokuyo</uri>
      <pose>0.2 0 0.2 0 0 0</pose>
    </include>
    <joint name="hokuyo_joint" type="revolute">
      <child>hokuyo::link`</child>
      <parent>chassis</parent>
      <axis>
        <xyz>0 0 1</xyz>
        <limit>
          <upper>0</upper>
          <lower>0</lower>
        </limit>
      </axis>
    </joint>
  </model>
</sdf>