Fix crash opening files saved in sculpt/vertex paint modes

Skip access to any evaluated data when operator is run on file load,
we don't have depsgraph evaluated yet. In this case we skip part of
sculpt session initialization, since it will be done during depsgraph
evaluation which happens after DEG_on_visible_update().

We can not skip sculpt session initialization since during normal
operation we want all the data to be initialized on mode change,
and not on initial brush stroke.

source/blender/blenkernel/BKE_context.h

@@ -322,6 +322,11 @@ int CTX_data_editable_gpencil_strokes(const bContext *C, ListBase *list);
 
 struct Depsgraph *CTX_data_depsgraph(const bContext *C);
 
+/* Will Return NULL if depsgraph is not allocated yet.
+ * Only used by handful of operators which are run on file load.
+ */
+struct Depsgraph *CTX_data_depsgraph_on_load(const bContext *C);
+
 #ifdef __cplusplus
 }
 #endif

source/blender/blenkernel/intern/context.c

@@ -1244,3 +1244,10 @@ Depsgraph *CTX_data_depsgraph(const bContext *C)
 	ViewLayer *view_layer = CTX_data_view_layer(C);
 	return BKE_scene_get_depsgraph(scene, view_layer, true);
 }
+
+Depsgraph *CTX_data_depsgraph_on_load(const bContext *C)
+{
+	Scene *scene = CTX_data_scene(C);
+	ViewLayer *view_layer = CTX_data_view_layer(C);
+	return BKE_scene_get_depsgraph(scene, view_layer, false);
+}

source/blender/blenkernel/intern/paint.c

@@ -865,6 +865,15 @@ void BKE_sculpt_update_mesh_elements(
         Depsgraph *depsgraph, Scene *scene, Sculpt *sd, Object *ob,
         bool need_pmap, bool need_mask)
 {
+	if (depsgraph == NULL) {
+		/* Happens on file load.
+		 *
+		 * We do nothing in this case, it will be taken care about on depsgraph
+		 * evaluation.
+		 */
+		return;
+	}
+
 	DerivedMesh *dm;
 	SculptSession *ss = ob->sculpt;
 	Mesh *me = ob->data;

source/blender/editors/sculpt_paint/paint_vertex.c

@@ -2342,7 +2342,7 @@ static int vpaint_mode_toggle_exec(bContext *C, wmOperator *op)
 		ED_object_vpaintmode_exit_ex(ob);
 	}
 	else {
-		Depsgraph *depsgraph = CTX_data_depsgraph(C);
+		Depsgraph *depsgraph = CTX_data_depsgraph_on_load(C);
 		wmWindowManager *wm = CTX_wm_manager(C);
 		ED_object_vpaintmode_enter_ex(depsgraph, wm, scene, ob);
 	}

source/blender/editors/sculpt_paint/sculpt.c

@@ -5793,7 +5793,7 @@ void ED_object_sculptmode_exit(bContext *C)
 
 static int sculpt_mode_toggle_exec(bContext *C, wmOperator *op)
 {
-	Depsgraph *depsgraph = CTX_data_depsgraph(C);
+	Depsgraph *depsgraph = CTX_data_depsgraph_on_load(C);
 	Scene *scene = CTX_data_scene(C);
 	Object *ob = CTX_data_active_object(C);
 	const int mode_flag = OB_MODE_SCULPT;