Handle null Styles when loading surface styles

An IfcSurfaceStyle with a NULL Styles set raised a TypeError when
loading a project. Guard the iterations, matching the existing
defensive pattern in import_presentation_styles.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/style.py

@@ -190,7 +190,7 @@ class Style(bonsai.core.tool.Style):
         else:
             style = blender_material_or_style
         style_elements = {}
-        for style_ in style.Styles:
+        for style_ in style.Styles or []:
             style_elements[style_.is_a()] = style_
         return style_elements
 
@@ -516,14 +516,14 @@ class Style(bonsai.core.tool.Style):
     @classmethod
     def get_surface_shading_style(cls, obj: bpy.types.Material) -> Union[ifcopenshell.entity_instance, None]:
         if style := tool.Ifc.get_entity(obj):
-            items = [s for s in style.Styles if s.is_a() == "IfcSurfaceStyleShading"]
+            items = [s for s in (style.Styles or []) if s.is_a() == "IfcSurfaceStyleShading"]
             if items:
                 return items[0]
 
     @classmethod
     def get_surface_texture_style(cls, obj: bpy.types.Material) -> Union[ifcopenshell.entity_instance, None]:
         if style := tool.Ifc.get_entity(obj):
-            items = [s for s in style.Styles if s.is_a("IfcSurfaceStyleWithTextures")]
+            items = [s for s in (style.Styles or []) if s.is_a("IfcSurfaceStyleWithTextures")]
             if items:
                 return items[0]
 

src/bonsai/test/tool/test_style.py

@@ -393,6 +393,19 @@ class TestGetUVMaps(NewFile):
         assert subject.get_uv_maps(representation) == [uv_map]
 
 
+class TestGetStyleElements(NewFile):
+    def test_style_with_null_styles(self):
+        style = ifcopenshell.file().create_entity("IfcSurfaceStyle", "Name", "BOTH", None)
+        assert subject.get_style_elements(style) == {}
+
+    def test_material_with_null_styles(self):
+        tool.Ifc.set(ifc := ifcopenshell.file())
+        style = ifc.create_entity("IfcSurfaceStyle", "Name", "BOTH", None)
+        material = bpy.data.materials.new("Material")
+        tool.Ifc.link(style, material)
+        assert subject.get_style_elements(material) == {}
+
+
 class TestImportSurfaceAttributes(NewFile):
     def test_run(self):
         tool.Ifc.set(ifc := ifcopenshell.file())