Gazebo GUI not save model custom collision

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I open a my_task.world file with following command,

$ gazebo my_task.world

In my my_task.world, there is a model with custom collision, like

<collision name="collision">
  <geometry>
        <mesh>
             <uri>model://visual/taskpad.dae</uri>
        </mesh>
   </geometry>
</collision>

<visual name="visual">
      <geometry>
        <mesh>
        <uri>model://visual/taskpad.dae</uri>
        </mesh>
      </geometry>
 </visual>

then I used Model Editor add a box in this world, and save & exit the Model Editor, and then save the world with gazebo GUI menu File -->Save World As.

Then I open the new .world file, but the custom collision with custom mesh was missing, and the collision property of the model not show up in the left tab page.