laser plug-in in urdf fails to publish scan results

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I have added differential drive plug-in and laser sensor (hokuyo) plug-in to my urdf file. I launched the robot in environment shown below and moved the robot using tele-operation.

Then i launched the rviz and did the setup required to view the scan results fron laser plug-in but it failed (refer the pic below).

Following are my terminal result and result for rostopic echo /scan respectively.

Below is my plug-in code

Link & joint

<link name="hokuyo_link">
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.1 0.1 0.1"/>
      </geometry>
    </collision>
    <visual>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.1 0.1 0.1"/>
      </geometry>
    </visual>
    <inertial>
      <mass value="1e-5" />
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <inertia ixx="1e-6" ixy="0" ixz="0" iyy="1e-6" iyz="0" izz="1e-6" />
    </inertial>
  </link>

  <joint name="hokuyo_joint" type="fixed">
    <axis xyz="0 1 0" />
    <origin xyz="0 -0.055 0.2" rpy="0 0 0"/>
    <parent link="Base_plate"/>
    <child link="hokuyo_link"/>
  </joint>

Laser controller

<gazebo reference="hokuyo_link">
    <sensor type="gpu_ray" name="hokuyo">
      <pose>0 0 0 0 0 0</pose>
      <visualize>false</visualize>
      <update_rate>40</update_rate>
      <ray>
        <scan>
          <horizontal>
            <samples>100</samples>
            <resolution>1</resolution>
            <min_angle>-1.570796</min_angle>
            <max_angle>1.570796</max_angle>
          </horizontal>
        </scan>
        <range>
          <min>0.10</min>
          <max>30.0</max>
          <resolution>0.01</resolution>
        </range>
      </ray>
      <plugin name="gpu_laser" filename="libgazebo_ros_gpu_laser.so">
        <topicName>/scan</topicName>
        <frameName>hokuyo_link</frameName>
      </plugin>
    </sensor>
  </gazebo>

How to fix the prblem and obtain the results from laser plug-in?