TechDraw: Crash when adding dimension

### Problem description

When adding a dimension FreeCAD crashes.

https://github.com/user-attachments/assets/e58f3acc-3f3b-44c9-8ebb-c5c7c2a020dd

When adding a dimension between the two points, see video, the application crashes.

### Workbench affected?

TechDraw

### Steps to reproduce

[Garagen Loch.zip](https://github.com/user-attachments/files/27557969/Garagen.Loch.zip)

1. open file
2. switch to TechDraw bench
3. Select the two dots, see video
4. Press d to create a dimension
5. place dimension

### Expected behavior

No Crash

### Actual behavior

Application crashes

### Development version About Info (in Safe Mode)

```shell
OS: KDE Flatpak runtime (GNOME/gnome/wayland)
Architecture: x86_64
Version: 1.1.1.44874 (Git)
Build date: 2026/04/14 22:09:59
Build type: Release
Branch: (HEAD detached at 1.1.1)
Hash: 0108fd4b4850cc46e625b60e53cea7a7bbe69f8d
Python 3.13.12, Qt 6.10.2, Coin 4.0.8, Vtk 9.4.2, boost 1_86, Eigen3 3.4.1, PySide 6.10.1
shiboken 6.10.1, SMESH 7.7.1.0, xerces-c 3.3.0, IfcOpenShell 0.0.0, OCC 7.8.1
Locale: German/Germany (de_DE)
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Dark/
Logical DPI/Physical DPI/Pixel Ratio: 96/169.808/1
```

### Last known good version (optional)

```shell

```

Start by opening the attached Garagen Loch.zip in FreeCAD, switching to the TechDraw workbench, and following the five reproduction steps. Confirm the crash occurs while creating and placing a dimension between the selected points, then trace the TechDraw dimension-creation path and verify that the same workflow completes without crashing after the fix.

Written by the indexing model from the issue text.