Exporting as VRM, getting error codes relating to materials?

I think I figured this problem out, but it took me a bit of digging. I've split this into a lengthy description of how I examined the problem, and then a brief description of how I believe @Halppls can fix it in their Blend file.

--Investigation/Explanation of Problem--

Following on my comments above, I ran this for loop (a slight modification of the one in the linked repo, focusing on the list that appears to contain a None value where a material is expected) in a Blender Scripting window:

>>> for mesh in [obj for obj in bpy.data.objects if obj.type == "MESH"]: ... mesh_vertex_group_names = [g.name for g in mesh.vertex_groups] ... for mat in mesh.data.materials: ... if mat is None: ... print(f"For {mesh}, got mat {mat}") 

This printed:

For <bpy_struct, Object("Object.037")>, got mat None For <bpy_struct, Object("Object.038")>, got mat None For <bpy_struct, Object("Object.039")>, got mat None For <bpy_struct, Object("Object.040")>, got mat None For <bpy_struct, Object("Object.041")>, got mat None For <bpy_struct, Object("Object.042")>, got mat None ... [ellipses mine; all objects between these two numbers were printed] For <bpy_struct, Object("Object.067")>, got mat None For <bpy_struct, Object("Object.068")>, got mat None For <bpy_struct, Object("Object.069")>, got mat None 

Selecting just Object.037 (which is the main, red part of the robot's body), I can verify that data.materials contains only the None material:

>>> [m for m in bpy.context.object.data.materials] [None] 

This is surprising, since Object.037, for example, clearly has RED set as its material. But, this is only true in the Materials sub-menu for Object -- i.e., it only has RED as a material linked to its object, not as a material linked to the data (meshes, curves, etc) making up the object. You can see this if you open this sub-menu:

Although you'll see that there is a material linked to the Object, switch that sub-menu to Data view, and you can see that there is only an empty materials slot on all of these objects. If we assign RED as a Data-linked material, now, then it looks like we get the behavior the AddOn expects:

>>> [m for m in bpy.context.active_object.data.materials] [bpy.data.materials['RED']] 

As an aside, this makes me wonder what the proper way to access Object-linked materials through the Python API is. It looks like this (which is not the way the VRM AddOn tries to access materials; it uses object.data.materials, which seems to only collect data-linked materials) kind of call may work, for accessing Object-linked materials:

>>> bpy.context.object.material_slots[0].material bpy.data.materials['ORANGE'] 

More information on the distinction between Object and Data linked materials can be found in the relevant Blender docs:

Specifies whether the material is to be linked to the Object or to the Object Data.

The Link selector has two choices, Data and Object. These two menu choices determine whether the material is linked to the object or to the data, (e.g. a mesh or curve). The Data menu item determines that this material will be linked to the mesh’s data-block which is linked to the object’s data-block. The Object menu item determines that the material will be linked to the object’s data-block directly.

--Just The Solution--

I think you just need to go through each of objects numbered 37 to 69, go into the Materials tab, switch from Object to Data linked view, and assign the desired material(s). After this, the VRM AddOn shouldn't see None as a material on these objects any longer.