From 5f7f6b65f3d88cb2952847de06e852062f9904de Mon Sep 17 00:00:00 2001
From: Bastien Montagne <bastien@blender.org>
Date: Fri, 6 Oct 2023 20:15:03 +0200
Subject: [PATCH] Fix (unreported) crash in 'startup.blend' GP default update
 code.

This commit fixes two different issues in
`BLO_update_defaults_startup_blend`:
* No check were done whether a given 'paint mode' tool settings actually
  exist or not before trying to 'update' it, leading to hard crash when
  some did not.
* Only the first Scene in loaded 'startup' Main would be processed -
  nothing prevents a `startup.blend` file to have more than one scene!
---
 .../blenloader/intern/versioning_defaults.cc  | 42 ++++++++++++-------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/source/blender/blenloader/intern/versioning_defaults.cc b/source/blender/blenloader/intern/versioning_defaults.cc
index d9245f77bde..b07bb4bd5d5 100644
--- a/source/blender/blenloader/intern/versioning_defaults.cc
+++ b/source/blender/blenloader/intern/versioning_defaults.cc
@@ -475,23 +475,37 @@ void BLO_update_defaults_startup_blend(Main *bmain, const char *app_template)
     }
 
     /* Reset all grease pencil brushes. */
-    Scene *scene = static_cast<Scene *>(bmain->scenes.first);
-    BKE_brush_gpencil_paint_presets(bmain, scene->toolsettings, true);
-    BKE_brush_gpencil_sculpt_presets(bmain, scene->toolsettings, true);
-    BKE_brush_gpencil_vertex_presets(bmain, scene->toolsettings, true);
-    BKE_brush_gpencil_weight_presets(bmain, scene->toolsettings, true);
+    LISTBASE_FOREACH (Scene *, scene, &bmain->scenes) {
+      ToolSettings *ts = scene->toolsettings;
 
-    /* Ensure new Paint modes. */
-    BKE_paint_ensure_from_paintmode(scene, PAINT_MODE_VERTEX_GPENCIL);
-    BKE_paint_ensure_from_paintmode(scene, PAINT_MODE_SCULPT_GPENCIL);
-    BKE_paint_ensure_from_paintmode(scene, PAINT_MODE_WEIGHT_GPENCIL);
+      if (ts->gp_paint) {
+        BKE_brush_gpencil_paint_presets(bmain, ts, true);
+      }
+      if (ts->gp_sculptpaint) {
+        BKE_brush_gpencil_sculpt_presets(bmain, ts, true);
+      }
+      if (ts->gp_vertexpaint) {
+        BKE_brush_gpencil_vertex_presets(bmain, ts, true);
+      }
+      if (ts->gp_weightpaint) {
+        BKE_brush_gpencil_weight_presets(bmain, ts, true);
+      }
 
-    /* Enable cursor. */
-    GpPaint *gp_paint = scene->toolsettings->gp_paint;
-    gp_paint->paint.flags |= PAINT_SHOW_BRUSH;
+      /* Ensure new Paint modes. */
+      BKE_paint_ensure_from_paintmode(scene, PAINT_MODE_VERTEX_GPENCIL);
+      BKE_paint_ensure_from_paintmode(scene, PAINT_MODE_SCULPT_GPENCIL);
+      BKE_paint_ensure_from_paintmode(scene, PAINT_MODE_WEIGHT_GPENCIL);
 
-    /* Ensure Palette by default. */
-    BKE_gpencil_palette_ensure(bmain, scene);
+      /* Enable cursor. */
+      if (ts->gp_paint) {
+        ts->gp_paint->paint.flags |= PAINT_SHOW_BRUSH;
+      }
+
+      /* Ensure Palette by default. */
+      if (ts->gp_paint) {
+        BKE_gpencil_palette_ensure(bmain, scene);
+      }
+    }
   }
 
   /* For builtin templates only. */