Fix for #7861 and #7801: Fix error in assign_container for unloaded elements

Decomposed elements collected via get_decomposition may not have a
corresponding Blender object if they are not loaded into the scene.
Guard against None before calling collector.assign.

Regression introduced in ff35666ad.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/core/spatial.py

@@ -67,7 +67,8 @@ def assign_container(
     if products := [e for e in root_elements if spatial.can_contain(container, root_element)]:
         ifc.run("spatial.assign_container", products=products, relating_structure=container)
     for element in all_elements:
-        collector.assign(ifc.get_object(element))
+        if obj := ifc.get_object(element):
+            collector.assign(obj)
 
 
 def enable_editing_container(spatial: type[tool.Spatial], obj: bpy.types.Object) -> None: