Do not try to kill thumbnail generation job when destroying the cache
(which happens as part of destroying the scene) -- all the code
paths that destroy a scene already cancel outstanding WM jobs. And WM
itself might be gone at that point, so accessing a stale pointer to it
can lead to a crash.
Instead, fix the problem of "refresh sequencer can cause a crash" by
not destroying the thumbnail cache, but merely clearing it in
sequencer_refresh_all_exec.
Pull Request: https://projects.blender.org/blender/blender/pulls/127485
649cde421b