Layer panel (also mask/ transform etc.) and other operators are gone
when properties tab is pinned and gpv3 object is non-active.
To fix this, get grease pencil pointer from `button_context()`
(done in `grease_pencil_context()`).
Continuation of cd476226d8
Pull Request: https://projects.blender.org/blender/blender/pulls/128475
bd4f7acce5