Error: Error [parser.cc:581] Unable to find uri[model://ground_plane]

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HI everyone, I'm making my final year project using Summit XL, but im having some problems with Gazebo launching. When I execute the command: roslaunch summit_xl_sim_bringup summit_xl_complete.launch Then gazebo and rviz open, but there's no world and the robot seems to sink slowly and this errors appear in the command window: Error [parser.cc:581] Unable to find uri[model://sun] Error [parser.cc:581] Unable to find uri[model://ground_plane] Error [parser.cc:581] Unable to find uri[model://willowgarage] How could i fix it? :)