Error opening .world file (the scene of the GUI remains black)

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Hi there, I've correctly installed Gazebo 11 on a virtual machine with Ubuntu.

I've followed the STEP 1 of the tutorial about how to create a Velodyne lidar system (this is the link http://gazebosim.org/tutorials?cat=gu...) till the end, where I should open the file .world just created with the instruction gazebo velodyne.world -u.

Actually, what I get is that Gazebo opens but the scene remains completely black. If I run the simulation nothing changes. However, on the left panel in the section World I can correctly see all the expected models, that is "ground_plane" and "velodyne_hdl-32" with its associated links (base and top).

Can anybody help me, please?