Fix segfault when loading an assembly with no children from xml/xbf

cadquery/occ_impl/importers/assembly.py

@@ -373,14 +373,16 @@ def _process_label(lbl: TDF_Label, parent: AssemblyProtocol):
     if shape_tool.IsTopLevel(top_level_label) and shape_tool.IsAssembly_s(
         top_level_label
     ):
-        # Set the name of the top-level assembly to match the top-level label
-        name_attr = TDataStd_Name()
-        top_level_label.FindAttribute(TDataStd_Name.GetID_s(), name_attr)
-
-        # Manipulation of .objects is needed to maintain consistency
-        assy.objects.pop(assy.name)
-        assy.name = str(name_attr.Get().ToExtString())
-        assy.objects[assy.name] = assy
+        # Set the name of the top-level assembly to match the top-level label. An
+        # assembly without children has no name attribute after an xml/xbf round
+        # trip, and FindAttribute segfaults on a label that does not have it.
+        name = _get_name(top_level_label)
+
+        if name:
+            # Manipulation of .objects is needed to maintain consistency
+            assy.objects.pop(assy.name)
+            assy.name = name
+            assy.objects[assy.name] = assy
 
         if cq_color:
             assy.color = cq_color

tests/test_assembly.py

@@ -2603,3 +2603,20 @@ def test_name_geometries(tmpdir):
     assert len([l for l in lines if "top_face" in l]) == 2
     assert len([l for l in lines if "plane_" in l]) == 2
     assert len([l for l in lines if "seg_" in l]) == 3
+
+
+@pytest.mark.parametrize("kind", ["step", "xml", "xbf"])
+def test_assembly_without_children_roundtrip(kind, tmpdir):
+    """
+    An assembly holding a single shape and no children used to segfault when
+    loaded back from xml or xbf.
+    """
+
+    assy = cq.Assembly(box(1, 1, 1))
+
+    path = os.path.join(tmpdir, f"no_children.{kind}")
+    assy.export(path)
+
+    loaded = cq.Assembly.load(path)
+
+    assert loaded.toCompound().Volume() == approx(1)