Caused by uninitialized `ToolSettings` `GpPaint` [which was later
accessed in `BKE_gpencil_palette_ensure`].
Not 100% sure why `ToolSettings` `GpPaint` is properly initialized in a
default startup fille, but for some files, this was not the case (as in
the report)
See 22462fed00 for a similar commit.
Now initialize `ToolSettings` `GpPaint` (alongside `GpVertexPaint`) when
entering grease pencil vertex paint mode.
Should probably go into LTS releases as well.
Philipp Oeser