FEM: test failure: segfault in test_box_frequency

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Problem description

When running with the address sanitizer on I am getting a segmentation fault in FEM's unit tests:
```
...
FemGui::ViewProviderFemPostObject::updateVtk() ViewProviderFemPostObject.cpp:408
FemGui::ViewProviderFemPostObject::setDisplayMode(const char *) ViewProviderFemPostObject.cpp:383
Gui::ViewProviderDocumentObject::setActiveMode() ViewProviderDocumentObject.cpp:493
Gui::ViewProviderDocumentObject::onChanged(const App::Property *) ViewProviderDocumentObject.cpp:168
FemGui::ViewProviderFemPostObject::onChanged(const App::Property *) ViewProviderFemPostObject.cpp:870
App::Property::hasSetValue() Property.cpp:269
App::PropertyEnumeration::setPyObject(_object *) PropertyStandard.cpp:477
App::PropertyContainerPy::setCustomAttributes(const char *, _object *) PropertyContainerPyImp.cpp:704
App::PropertyContainerPy::_setattr(const char *, _object *) PropertyContainerPy.cpp:1216
App::ExtensionContainerPy::_setattr(const char *, _object *) ExtensionContainerPy.cpp:282
Gui::ViewProviderPy::_setattr(const char *, _object *) ViewProviderPy.cpp:2254
Gui::ViewProviderDocumentObjectPy::_setattr(const char *, _object *) ViewProviderDocumentObjectPy.cpp:379
FemGui::ViewProviderFemPostPipelinePy::_setattr(const char *, _object *) ViewProviderFemPostPipelinePy.cpp:301
Base::PyObjectBase::__setattro(_object *, _object *, _object *) PyObjectBase.cpp:385
[Inlined] _Py_Dealloc(_object *) object.c:2938
[Inlined] Py_DECREF(_object *) object.h:949
PyObject_SetAttr(_object *, _object *, _object *) object.c:1426
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:5492
_PyFunction_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:418
_PyObject_VectorcallTstate(_ts *, _object *, _object *const *, unsigned long long, _object *) pycore_call.h:168
method_vectorcall(_object *, _object *const *, unsigned long long, _object *) classobject.c:92
_PyVectorcall_Call(_ts *, _object *(*)(_object *, _object *const *, unsigned long long, _object *), _object *, _object *, _object *) call.c:273
_PyObject_Call(_ts *, _object *, _object *, _object *) call.c:348
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:1355
_PyFunction_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:418
_PyObject_VectorcallDictTstate(_ts *, _object *, _object *const *, unsigned long long, _object *) call.c:135
_PyObject_Call_Prepend(_ts *, _object *, _object *, _object *, _object *) call.c:504
slot_tp_call(_object *, _object *, _object *) typeobject.c:9565
_PyObject_MakeTpCall(_ts *, _object *, _object *const *, long long, _object *) call.c:242
PyObject_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:327
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:1843
_PyFunction_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:418
_PyObject_VectorcallTstate(_ts *, _object *, _object *const *, unsigned long long, _object *) pycore_call.h:168
method_vectorcall(_object *, _object *const *, unsigned long long, _object *) classobject.c:92
_PyVectorcall_Call(_ts *, _object *(*)(_object *, _object *const *, unsigned long long, _object *), _object *, _object *, _object *) call.c:273
_PyObject_Call(_ts *, _object *, _object *, _object *) call.c:348
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:1355
_PyFunction_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:418
_PyObject_VectorcallDictTstate(_ts *, _object *, _object *const *, unsigned long long, _object *) call.c:135
_PyObject_Call_Prepend(_ts *, _object *, _object *, _object *, _object *) call.c:504
slot_tp_call(_object *, _object *, _object *) typeobject.c:9565
_PyObject_MakeTpCall(_ts *, _object *, _object *const *, long long, _object *) call.c:242
PyObject_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:327
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:1843
_PyFunction_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:418
_PyObject_VectorcallTstate(_ts *, _object *, _object *const *, unsigned long long, _object *) pycore_call.h:168
method_vectorcall(_object *, _object *const *, unsigned long long, _object *) classobject.c:92
_PyVectorcall_Call(_ts *, _object *(*)(_object *, _object *const *, unsigned long long, _object *), _object *, _object *, _object *) call.c:273
_PyObject_Call(_ts *, _object *, _object *, _object *) call.c:348
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:1355
_PyFunction_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:418
_PyObject_VectorcallDictTstate(_ts *, _object *, _object *const *, unsigned long long, _object *) call.c:135
_PyObject_Call_Prepend(_ts *, _object *, _object *, _object *, _object *) call.c:504
slot_tp_call(_object *, _object *, _object *) typeobject.c:9565
_PyObject_MakeTpCall(_ts *, _object *, _object *const *, long long, _object *) call.c:242
PyObject_Vectorcall(_object *, _object *const *, unsigned long long, _object *) call.c:327
_PyEval_EvalFrameDefault(_ts *, _PyInterpreterFrame *, int) generated_cases.c.h:1843
[Inlined] _PyEval_EvalFrame(_ts *, _PyInterpreterFrame *, int) pycore_ceval.h:119
[Inlined] _PyEval_Vector(_ts *, PyFunctionObject *, _object *, _object *const *, unsigned long long, _object *) ceval.c:1816
PyEval_EvalCode(_object *, _object *, _object *) ceval.c:604
run_eval_code_obj(_ts *, PyCodeObject *, _object *, _object *) pythonrun.c:1381
run_mod(_mod *, _object *, _object *, _object *, PyCompilerFlags *, _arena *, _object *, int) pythonrun.c:1466
_PyRun_StringFlagsWithName(const char *, _object *, int, _object *, _object *, PyCompilerFlags *, int) pythonrun.c:1261
PyRun_StringFlags(const char *, int, _object *, _object *, PyCompilerFlags *) pythonrun.c:1273
Base::InterpreterSingleton::runString(const char *) Interpreter.cpp:246
TestGui::UnitTestDialog::onStartButtonClicked() UnitTestImp.cpp:192
`QtPrivate::FunctorCall,QtPrivate::List<>,void,void (__cdecl TestGui::UnitTestDialog::*)(void)>::call'::`2'::::operator()() qobjectdefs_impl.h:127
QtPrivate::FunctorCallBase::call_internal,QtPrivate::List<>,void,void (__cdecl TestGui::UnitTestDialog::*)(void)>::call'::`2':: >(void **, &&) qobjectdefs_impl.h:65
QtPrivate::FunctorCall,QtPrivate::List<>,void,void (__cdecl TestGui::UnitTestDialog::*)(void)>::call(void (TestGui::UnitTestDialog::*)(),TestGui::UnitTestDialog *,void **) qobjectdefs_impl.h:126
QtPrivate::FunctionPointer::call<…>(void (TestGui::UnitTestDialog::*)(), TestGui::UnitTestDialog *, void **) qobjectdefs_impl.h:174
QtPrivate::QCallableObject::impl(int, QtPrivate::QSlotObjectBase *, QObject *, void **, bool *) qobjectdefs_impl.h:545
0x00007ffb6442416f
0x00007ffb64426f74
0x00007ffb6f32cde7
0x00007ffb6f32ca9e
0x00007ffb6f32de5c
0x00007ffb6f265e04
0x00007ffb6f2236a6
0x00007ffb6f22180d
Gui::GUIApplication::notify(QObject *, QEvent *) GuiApplication.cpp:96
0x00007ffb643e4731
0x00007ffb6f22615e
0x00007ffb6f28c27a
0x00007ffb6f289b8d
0x00007ffb6f2236a6
0x00007ffb6f2227d6
Gui::GUIApplication::notify(QObject *, QEvent *) GuiApplication.cpp:96
0x00007ffb643e4731
0x00007ffb64d5bcaf
0x00007ffb64db919b
0x00007ffb64572c39
0x00007ffb6507bb79
0x00007ffb643eb1a4
0x00007ffb643e2dfa
`anonymous namespace'::tryRunEventLoop(Gui::GUISingleApplication &) Application.cpp:2341
`anonymous namespace'::runEventLoop(Gui::GUISingleApplication &) Application.cpp:2370
Gui::Application::runApplication() Application.cpp:2482
0x00007ff674b44fa3
0x00007ff674be2540
0x00007ff674be19f6
0x00007ffc4acbe8d7
0x00007ffc4b9ac53c
```

### Full version info

```shell
OS: Windows 11 build 26100
Architecture: x86_64
Version: 1.1.0dev.44073 (Git)
Build date: 2025/10/28 20:29:01
Build type: Unknown
Branch: main
Hash: 5b2987bd0751d5ee7ab95535367807fce54e803a
Python 3.13.5, Qt 6.9.1, Coin 4.0.4, Vtk 9.5.0, boost 1_88, Eigen3 3.4.0, PySide 6.9.1
shiboken 6.9.1, SMESH 7.7.1.0, xerces-c 3.2.5, OCC 7.8.1
Locale: English/United States (en_US)
Navigation Style/Orbit Style/Rotation Mode: OpenSCAD/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
Logical DPI/Physical DPI/Pixel Ratio: 96/107.975/1.5
Installed mods:
* AddonManager 2025.8.23dev
```

### Subproject(s) affected?

None

### Anything else?

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct