This was very simple to reproduce, just turn on Freestyle and press render.
Now to the truth of things. Most (if not all) of
~BlenderStrokeRenderer() can be removed. I believe this was done back
when freestyle was using G.main, and since we gave freestyle its own
main we can just leave the cleanup for later.
I will leave this for freestyle maintainers to think over though.
Note: There is a chance this was the issue reported on T57890. I will
wait for the reporter to confirm this as fixed though.
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