Fix crash after undo of assign_class on macOS (#7419)

After assigning an IFC class and undoing, msgbus subscriptions registered
with the old Python object wrapper survived (PERSISTENT flag) but could
not be cleared because: (1) rollback_link_element looked up objects by
their post-link name which no longer exists after undo, and (2) the
per-object clear_by_owner calls in rebuild_element_maps used new Python
wrappers that didn't match the old subscription owners.

Fix by using a dedicated stable object (object_subscription_owner) as
the msgbus owner for all per-object subscriptions, allowing
rebuild_element_maps to clear all stale subscriptions in one call
regardless of Python wrapper identity changes during undo/redo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/bonsai/bonsai/bim/handler.py

@@ -45,16 +45,19 @@ from bonsai.bim.module.nest.decorator import NestDecorator
 
 cwd = os.path.dirname(os.path.realpath(__file__))
 global_subscription_owner = object()
+# Separate owner for per-object msgbus subscriptions (name, active_material_index).
+# Using a dedicated owner allows clearing all per-object subscriptions at once
+# during undo/redo without affecting other global subscriptions.
+object_subscription_owner = object()
 
 
 def name_callback(obj: Union[bpy.types.Object, bpy.types.Material], data: str) -> None:
     try:
         obj.name
     except:
-        # The object is invalid but somehow still has a callback. Clear all
+        # The object is invalid but somehow still has a callback.
-        # msgbus subscriptions to prevent useless further triggers.
+        # This can occur during undo/redo when the Python wrapper is stale.
-        bpy.msgbus.clear_by_owner(obj)
+        return
-        return  # In case the object RNA is gone during an undo / redo operation
     # Blender names are up to 63 UTF-8 bytes
     if len(bytes(obj.name, "utf-8")) >= 63:
         return
@@ -189,7 +192,7 @@ def subscribe_to(obj: bpy.types.ID, data_path: str, callback: Callable[[bpy.type
         return
     bpy.msgbus.subscribe_rna(
         key=subscribe_to,
-        owner=obj,
+        owner=object_subscription_owner,
         args=(
             obj,
             data_path,

src/bonsai/bonsai/bim/ifc.py

@@ -316,11 +316,8 @@ class IfcStore:
         del IfcStore.id_map[data["id"]]
         if "guid" in data:
             del IfcStore.guid_map[data["guid"]]
-        obj = IfcStore.get_object_by_name(data["obj"])
+        # Note: msgbus subscriptions are cleared globally during
-        if obj is None:
+        # rebuild_element_maps which runs after every undo/redo.
-            # obj was just created during this step and didn't existed before.
-            return
-        bpy.msgbus.clear_by_owner(obj)
 
     @staticmethod
     def commit_link_element(data: OperationData) -> None:
@@ -367,10 +364,8 @@ class IfcStore:
         del IfcStore.id_map[data["id"]]
         if "guid" in data:
             del IfcStore.guid_map[data["guid"]]
-        obj = IfcStore.get_object_by_name(data["obj"])
+        # Note: msgbus subscriptions are cleared globally during
-        # obj might be removed after unlink.
+        # rebuild_element_maps which runs after every undo/redo.
-        if not obj:
-            bpy.msgbus.clear_by_owner(obj)
 
     @staticmethod
     def unlink_element(

src/bonsai/bonsai/tool/ifc.py

@@ -197,12 +197,16 @@ class Ifc(bonsai.core.tool.Ifc):
         if not cls.get():
             return
 
+        # Clear all per-object msgbus subscriptions at once using the dedicated
+        # owner.  After undo/redo, per-object Python wrappers have new
+        # identities so clearing by individual obj would miss stale
+        # subscriptions registered with the old wrappers.
+        bpy.msgbus.clear_by_owner(bonsai.bim.handler.object_subscription_owner)
+
         for obj in bpy.data.objects:
             if obj.library:
                 continue
 
-            bpy.msgbus.clear_by_owner(obj)
-
             element = cls.get_entity(obj)
             if not element:
                 continue
@@ -217,8 +221,6 @@ class Ifc(bonsai.core.tool.Ifc):
             if obj.library:
                 continue
 
-            bpy.msgbus.clear_by_owner(obj)
-
             style = cls.get_entity(obj)
             if not style:
                 continue