# Walkthrough: Fix Preview Render TypeError

I have fixed the `TypeError: cannot unpack non-iterable GeologySegment object` that occurred when updating the preview with geology data but no topography data.

## 🛠️ Problem Identification
The error occurred in `gui/preview_renderer.py` because the code was attempting to unpack `GeologySegment` objects as if they were 3-tuples `(distance, elevation, unit)`. This was a legacy part of the code that wasn't updated when `GeologyData` was refactored to use `GeologySegment` dataclasses.

```python
# Failing code
reference_data = (
    topo_data if topo_data else ([(d, e) for d, e, _ in geol_data] if geol_data else None)
)
```

## 🚀 Solution implementation
I updated the list comprehension to correctly extract the points from each `GeologySegment` object.

```python
# Fixed code
reference_data = (
    topo_data
    if topo_data
    else ([p for seg in geol_data for p in seg.points] if geol_data else None)
)
```

## 🧪 Verification Results

### Regression Test
I created a specific test case to reproduce the error by providing `geol_data` but `topo_data=None`.
The test now passes without error.

### Automated Tests
I integrated the regression test into the main test suite:
- [test_preview_components.py](file:///home/jmbernales/qgispluginsdev/sec_interp/tests/gui/test_preview_components.py)

**Result**: All 316 tests passed.

```bash
PYTHONPATH=.. uv run python3 -m unittest discover tests
# Ran 316 tests in 0.555s
# OK (skipped=4)
```
