Handling null TDF Label - Fix Issue #618

src/build123d/exporters3d.py

@@ -135,7 +135,7 @@ def _create_xde(to_export: Shape, unit: Unit = Unit.MM) -> TDocStd_Document:
                 shape_tool.FindShape(sub_node, findInstance=False)
                 for sub_node in sub_nodes
             ]
-        if node.label:
+        if node.label and not node_label.IsNull():
             TDataStd_Name.Set_s(node_label, TCollection_ExtendedString(node.label))
 
         if node.color is not None:

tests/test_exporters3d.py

@@ -31,6 +31,7 @@
 import unittest
 from typing import Optional
 
+from bd_warehouse.thread import TrapezoidalThread
 from build123d.build_common import GridLocations
 from build123d.build_enums import Unit
 from build123d.build_line import BuildLine
@@ -141,6 +142,18 @@ def test_export_step_unknown(self):
         os.chmod("box_read_only.step", 0o777)  # Make the file read/write
         os.remove("box_read_only.step")
 
+    def test_export_step_null_label(self):
+        # Moving the object creates a null TDF_Label
+        testobj = TrapezoidalThread(
+            diameter=20,
+            pitch=1.5,
+            length=20,
+            thread_angle=30.0,
+        )
+        testobj_moved = testobj.moved(Pos(0, 0, 10))
+        self.assertTrue(export_step(testobj_moved, "test.step"))
+        os.remove("test.step")
+
 
 class TestExportGltf(DirectApiTestCase):
     def test_export_gltf(self):