I launch Gazebo and when I ask it to exit, just seconds after it finished opening the window, it seems tonot want to exit. The launch looks like this:

roslaunch turtlebot3_gazebo turtlebot3_world.launch

Everything seems ok, like this:

... logging to /home/pitosalas/.ros/log/cc98a286-2b38-11e9-a017-002500efe1ca/roslaunch-pitosalasubuntu-2085.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://localhost:46757/

SUMMARY
========

PARAMETERS
 * /robot_description: <?xml version="1....
 * /rosdistro: kinetic
 * /rosversion: 1.12.14
 * /use_sim_time: True

NODES
  /
    gazebo (gazebo_ros/gzserver)
    gazebo_gui (gazebo_ros/gzclient)
    spawn_urdf (gazebo_ros/spawn_model)

auto-starting new master
process[master]: started with pid [2099]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to cc98a286-2b38-11e9-a017-002500efe1ca
process[rosout-1]: started with pid [2112]
started core service [/rosout]
process[gazebo-2]: started with pid [2115]
process[gazebo_gui-3]: started with pid [2120]
process[spawn_urdf-4]: started with pid [2125]
[ INFO] [1549585858.269073330]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1549585858.269757091]: waitForService: Service [/gazebo/set_physics_properties] has not been advertised, waiting...
[ INFO] [1549585858.301470474]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1549585858.302111616]: waitForService: Service [/gazebo/set_physics_properties] has not been advertised, waiting...
[ INFO] [1549585859.229039248, 0.023000000]: waitForService: Service [/gazebo/set_physics_properties] is now available.
[ INFO] [1549585859.928332791, 0.113000000]: Physics dynamic reconfigure ready.
[ INFO] [1549585859.937520292, 0.113000000]: Laser Plugin: Using the 'robotNamespace' param: '/'
[ INFO] [1549585859.937585152, 0.113000000]: Starting Laser Plugin (ns = /)
[ INFO] [1549585859.939827688, 0.113000000]: Laser Plugin (ns = /)  <tf_prefix_>, set to ""
[ INFO] [1549585860.021103271, 0.113000000]: Starting plugin DiffDrive(ns = //)
[ INFO] [1549585860.021237533, 0.113000000]: DiffDrive(ns = //): <rosDebugLevel> = na
[ INFO] [1549585860.022563155, 0.113000000]: DiffDrive(ns = //): <tf_prefix> = 
[ INFO] [1549585860.024129803, 0.113000000]: DiffDrive(ns = //): Advertise joint_states
[ INFO] [1549585860.025187518, 0.113000000]: DiffDrive(ns = //): Try to subscribe to cmd_vel
[ INFO] [1549585860.029586139, 0.113000000]: DiffDrive(ns = //): Subscribe to cmd_vel
[ INFO] [1549585860.030441632, 0.113000000]: DiffDrive(ns = //): Advertise odom on odom 
[ INFO] [1549585860.061165409, 0.135000000]: waitForService: Service [/gazebo/set_physics_properties] is now available.

Then I exit with the menu command, and it sits here:

[ INFO] [1549585860.108192291, 0.181000000]: Physics dynamic reconfigure ready.
[spawn_urdf-4] process has finished cleanly
log file: /home/pitosalas/.ros/log/cc98a286-2b38-11e9-a017-002500efe1ca/spawn_urdf-4*.log
[gazebo_gui-3] process has finished cleanly
log file: /home/pitosalas/.ros/log/cc98a286-2b38-11e9-a017-002500efe1ca/gazebo_gui-3*.log

For ages. Another shell shows this:

pitosalas@pitosalasubuntu:~$ ps ax | grep gaz
 2085 pts/18   Sl+    0:00 /usr/bin/python /opt/ros/kinetic/bin/roslaunch turtlebot3_gazebo turtlebot3_world.launch
 2115 ?        Ss     0:00 /bin/sh /opt/ros/kinetic/lib/gazebo_ros/gzserver -e ode /home/pitosalas/catkin_ws/src/turtlebot3_simulations/turtlebot3_gazebo/worlds/turtlebot3_world.world __name:=gazebo __log:=/home/pitosalas/.ros/log/cc98a286-2b38-11e9-a017-002500efe1ca/gazebo-2.log
 2183 ?        Sl     1:07 gzserver -e ode /home/pitosalas/catkin_ws/src/turtlebot3_simulations/turtlebot3_gazebo/worlds/turtlebot3_world.world -s /opt/ros/kinetic/lib/libgazebo_ros_paths_plugin.so -s /opt/ros/kinetic/lib/libgazebo_ros_api_plugin.so __name:=gazebo __log:=/home/pitosalas/.ros/log/cc98a286-2b38-11e9-a017-002500efe1ca/gazebo-2.log
 2811 pts/1    S+     0:00 grep --color=auto gaz