Hey,
I have created a hello.world file. The simplemobilemanipulator is a pioneer2dx model attached with a hokuyo lidar. I have added the plugin in the hokuyo model to be able to see the lidar data.
When I roslaunch the launch file, this world opens correctly. When I see the data of the hokuyo lidar on rviz, there is a error: For frame [hokuyo_link]: Fixed Frame [map] does not exist on rviz

How to solve this issue?

This is the hello.world file. <?xml version="1.0" ?>

<include>
  <uri>model://ground_plane</uri>
</include>

<include>
  <uri>model://sun</uri>
</include>

<include>
  <uri>model://simple_mobile_manipulator</uri>
  <pose>0 0 0 0 0 0</pose>
</include>

<include>
  <uri>model://pioneer2dx</uri>
  <pose>2 0 0 0 0 0</pose>
</include>

<!-- reference to your plugin -->
<plugin name="gazebo_tutorials" filename="libgazebo_tutorials.so"/>