System Information
Operating system: Gentoo Linux
Graphics card: VEGA 64

Blender Version
Broken: Master

Short description of error
This only happens in debug builds.

If you close (or open a new file) when you have had a split with a thumbnail preview in it. The following assert will be triggered:
BLI_assert failed: source/blender/editors/space_file/space_file.c:163, file_free(), at 'sfile->previews_timer == ((void *)0)'

Exact steps for others to reproduce the error

1. Open Blender
2. Open the "Video Editing" template (New -> Video Editing)
3. Switch the filebrowser tab to 3D view
4. Close Blender or open an other file
5. Blender asserts and closes