RNA: Make is clear that `Scene` parameter of `update` callback may be NULL.

There are cases where there is no way to ensure we do have/know about an
active scene. Further more, this should not be required to perform
'real' updates on data, only to perform additional special handling in
current scene (mostly related to editing tools, UI, etc.).

This pointer is actually almost never used in practice, and half of its current
 usages are fairly close to abuse of the system (like calls to
`ED_gpencil_tag_scene_gpencil` or `BKE_rigidbody_cache_reset`).

This commit ensures that the few places using this 'active scene' pointer are
safely handling the `NULL` case, and clearly document the fact that a
NULL scene pointer is valid.

1 files changed, 3 insertions, 1 deletions

diff --git a/source/blender/makesrna/intern/rna_scene.c b/source/blender/makesrna/intern/rna_scene.c
index ba5b3095996..00b7f0c9106 100644
--- a/source/blender/makesrna/intern/rna_scene.c
+++ b/source/blender/makesrna/intern/rna_scene.c
@@ -676,7 +676,9 @@ static void rna_ToolSettings_snap_mode_set(struct PointerRNA *ptr, int value)
 /* Grease Pencil update cache */
 static void rna_GPencil_update(Main *UNUSED(bmain), Scene *scene, PointerRNA *UNUSED(ptr))
 {
-  ED_gpencil_tag_scene_gpencil(scene);
+  if (scene != NULL) {
+    ED_gpencil_tag_scene_gpencil(scene);
+  }
 }
 
 static void rna_Gpencil_extend_selection(bContext *C, PointerRNA *UNUSED(ptr))