gazebo does not load simple model

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I created a simple .world file:

<?xml version="1.0"?>
<sdf version="1.4">
    <world name="office">
        <include>
            <uri>model://sun</uri>
        </include>

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

My launch file also adds a robot to this world. It launches fine in gazebo, I can see the gray ground plane with grid markings on it and also my robot.

Next I created a simple building layout (some walls created using Gazebo's building editor). I saved the building model and added these lines to my .world file:

<include>
    <uri>model://car_description/models/tunnel</uri>
    <pose>25 20.2 0 0 0 0</pose>
</include>

When I launch the file again, this time it takes ages to launch. I see a 'Preparing your world ...' screen for a long time (~10 mins):

and then I see this dark screen:

Other solutions which I have tried and did not work for me are:

1. Adding the following code to the package's package.xml

   <export>
      <gazebo_ros gazebo_model_path="${prefix}/models"/>
      <gazebo_ros gazebo_media_path="${prefix}/models"/>
   </export>
   

2. Adding the environment variable GAZEBO_MODEL_PATHto my .bashrc file:

   export GAZEBO_MODEL_PATH=/home/my_user/my_catkin_ws/src/
   

There are no errors thrown in the terminal. What am I doing wrong?