MeshInstance3D::get_active_material() causes out-of-bounds error when surface_override_materials is empty

Tested versions

Reproducible in Godot 4.7.2.stable

System information

Windows 10 - Godot v4.7.2.stable - Vulkan (Forward+) - dedicated RX 580

Issue description

Bug: MeshInstance3D get_active_material() can access an empty surface_override_materials array

Description

MeshInstance3D::get_active_material(int p_surface) calls get_surface_override_material(p_surface) after checking that the Mesh has at least one surface.

However, surface_override_materials can have size 0 at the time of the call.

This results in:

ERROR: scene/3d/mesh_instance_3d.cpp:388 - Index p_surface = 0 is out of bounds (surface_override_materials.size() = 0)

Relevant code

Ref<Material> MeshInstance3D::get_active_material(int p_surface) const {
    Ref<Material> mat_override = get_material_override();
    if (mat_override.is_valid()) {
        return mat_override;
    }

    Ref<Mesh> m = get_mesh();
    if (m.is_null() || m->get_surface_count() == 0) {
        return Ref<Material>();
    }

    Ref<Material> surface_material = get_surface_override_material(p_surface);

    if (surface_material.is_valid()) {
        return surface_material;
    }

    return m->surface_get_material(p_surface);
}

get_surface_override_material() performs:

ERR_FAIL_INDEX_V(p_surface, surface_override_materials.size(), Ref<Material>());

At the time of the error:

p_surface = 0
surface_override_materials.size() = 0

Expected behavior

get_active_material(0) should safely return the mesh's material when there is no surface override, without producing an out-of-bounds error.

Actual behavior

An out-of-bounds error is printed because get_active_material() calls get_surface_override_material() while the override-material array is empty.

Steps to reproduce

1. [ضع الخطوات الدقيقة هنا]
2. [ضع الـ Mesh/Scene المستخدم هنا]
3. [ضع العملية التي تؤدي إلى استدعاء get_active_material() هنا]
4. Error appears in mesh_instance_3d.cpp.

Additional information

The surface_override_materials array is resized in _mesh_changed():

surface_override_materials.resize(surface_count);

This suggests the issue may involve a timing/state synchronization case where get_active_material() is called before the override array has been initialized or synchronized with the Mesh surface count.

A defensive bounds check inside get_active_material() may prevent the error, but the underlying cause of the array being empty should also be investigated.

Steps to reproduce

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Minimal reproduction project (MRP)

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