Trying to export with "Remove Armature Object" enabled causes "AttributeError: 'NoneType' object has no attribute 'children'" under certain conditions

[saturday06]

Describe the bug

Trying to export the armature and mesh as a GLB file while "Remove Armature Object" is enabled will result in an "AttributeError: 'NoneType' object has no attribute 'children'" and the export will fail under certain conditions.

22:14:22 | INFO: Starting glTF 2.0 export
22:14:22 | INFO: Extracting primitive: Triangle
22:14:22 | INFO: Primitives created: 1
Traceback (most recent call last):
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\__init__.py", line 1313, in execute
    res = gltf2_blender_export.save(context, export_settings)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_export.py", line 37, in save
    json, buffer = __export(export_settings)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_export.py", line 55, in __export
    __gather_gltf(exporter, export_settings)
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_export.py", line 202, in __gather_gltf
    active_scene_idx, scenes, animations = gltf2_blender_gather.gather_gltf2(export_settings)
                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_gather.py", line 31, in gather_gltf2
    scenes.append(__gather_scene(blender_scene, export_settings))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_gather_cache.py", line 37, in wrapper_cached
    result = func(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_gather.py", line 123, in __gather_scene
    vtree.add_neutral_bones()
  File "C:\Program Files\WindowsApps\BlenderFoundation.Blender4.2LTS_4.2.4.0_x64__ppwjx1n5r4v9t\Blender\4.2\scripts\addons_core\io_scene_gltf2\blender\exp\gltf2_blender_gather_tree.py", line 822, in add_neutral_bones
    self.nodes[n.armature].node.children.append(neutral_bone)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'children'

To Reproduce

1. Open the attached remove-armature-object-debug-20241210.blend
2. Go to the glTF exporter (File > Export > glTF 2.0 (.glb/.gltf))
3. Enable the "Remove Armature Object" option
4. Attempt to export the file

Expected behavior

The export should not fail.

Screenshots

remove-armature-object-debug-20241210.mp4

.blend file/ .gltf (mandatory)

remove-armature-object-debug-20241210.zip

Version

• OS: Windows 11 24H2
• Blender Version 4.2.4, 4.3.0

Additional context

Original bug report is saturday06/VRM-Addon-for-Blender#675

[julienduroure]

Hello,
Confirmed.
This happen only when some vertices are not assigned to any bones (so glTF exporter create a neutral joint to assign these vertices to it)

[julienduroure]

Well,

There is no easy fix about it:

• We check if we can remove the armature object at start of the process, without knowing yet if a neutral bone is required
• The addition of the neutral bone is calculated during the mesh/skin extraction
• If the armature is at root of the scene, we cannot add the neutral bone, as we must have in glTF a unique root joint (see error SKIN_NO_COMMON_ROOT in validator). So ... If we remove the armature and add a neutral bone, the glTF file is not valid

Investigation / thinking in progress