I have a problem with my maze, simply when I start the world it appears empty, even though I defined the location of the models inside the .world file in my ROS packages in Catkin's Workspace like </include> <url>model://my_maze</url> </include>, I tried to change the location of the folder where my package is to the gazebo folder and still the items do not load, just appears the turtlebot3 in a empty world, can someone help me, it would be for a final project at my college.