PR: Export IfcSlab problem - strange behavior

### Problem Description

We found a strange issue in export from Revit:
When exporting whole project there are some elements not being exported and we could not identify why.
Then we tried to isolate this element and found an issue → To describe and provide as much info as possible:

**Problematic elements**: Floors

- _IFC2x3 → Exported correctly_
- _IFC4 RV (Arch) → Exported correctly_
- _IFC4 RV (Struc) → Exported correctly_
- _IFC4 DTV → Exported correctly_
- **IFC4x3 → Error:**

Problematic element is a Floor with opening and slope. (File saved here to replicate issue https://1drv.ms/f/c/1f30c6c2d1dbf645/EjtXB-dAzIhMg4MFtG6B31gBd328a7ww6h5TMdv3vpUIow?e=UIEWNL)

We have narrowed down all eventualities and found there are 3 factors that are somehow affecting export to IFC4x3.
- **Slope**
- **Opening**
- **Material**

Problem is being caused by material **Default Floor**! The very moment you delete this material (despite not being assigned to the element) export is working perfectly. That is the most strange thing about this bug - why Material, that is not being assigned to element, is causing problems if floor is exported and only if the floor has a slope and opening??

Combinations to clarify behavior:

- Slope + Opening - (Material deleted) → OK
- Slope + Material - (Opening deleted) → OK
- Opening + Material - (Slope deleted) → OK
- **Slope + Opening + Material → Error**

Thankfully we have found a simple fix (delete material from project), but this should be fixed if possible.
To test it yourself download RVT file (v2024) and try to export to IFC4x3 → Error, if you then delete material "Default Floor" from Revit project → Export works miraculously.

### Revit Version

2024.3.x

### IFC for Revit Addon Version

24.2.2

### Windows Version

11 24H2

Download the linked RVT file and reproduce the IFC4x3 export failure using Revit 2024.3.x and IFC for Revit 24.2.2. Compare exports with slope, opening, and the unassigned Default Floor material present or deleted; done means the floor exports successfully with all three factors present.

Written by the indexing model from the issue text.