Fix (unreported) broken collections after undo/redo, or remapping (leading to crashes).

Those are two cases where keeping infamous backward `parents` pointers
of collections in sync is kind of impossible to do... So rebuilding
those relationships from scratch instead.

Fixes e.g. a crash when undoing, then reloading a library, and likely
many more weird ones like that.

Uncovered while investigating T64764.

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/library_remap.c b/source/blender/blenkernel/intern/library_remap.c
index 4e5eac7924b..e0e473978f8 100644
--- a/source/blender/blenkernel/intern/library_remap.c
+++ b/source/blender/blenkernel/intern/library_remap.c
@@ -368,6 +368,11 @@ static void libblock_remap_data_postprocess_collection_update(Main *bmain,
      * I'd consider optimizing that whole collection remapping process a TODO for later. */
     BKE_collections_child_remove_nulls(bmain, NULL /*old_collection*/);
   }
+  else {
+    /* Temp safe fix, but a "tad" brute force... We should probably be able to use parents from
+     * old_collection instead? */
+    BKE_main_collections_parent_relations_rebuild(bmain);
+  }
 
   BKE_main_collection_sync_remap(bmain);
 }