Gazebo crashing just after launch... also taking a long time to exit

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I put these together because I think they may be related. This is Gazebo 7 running on Ubuntu 16.04 with the Turtlebot3 libraries. And my installation is quite fresh.

When I try to exit Gazebo, both with a ^c and by using the exit command, often it takes many minutes to finish exiting. And even then it might not be all done.

I sometimes do a ps ax | grep gaz and kill -9 all the named processes and I think that solves it. But sometimes it seems like I need to also logout (although that might just be a superstition.)

Related, I think, sometimes when I run Gazebo it crashes hard soon after loading. I see the empty 3d view screen but the world doesnt seem to get displayed before it crashes.

Does anyone see this too, and is there an explanation?