Assembly: Fix crash on right click assembly link

[PaddleStroke]

Fix #29702

Note before cmd->getAction() never returned null. I think before cmd was null if there was no action.

So what happens is that if you have not loaded assembly workbench, then the command Assembly_LinkSelectLinked is not loaded yet.

Before I guess that in this case cmd was nullptr. But now it is apparantly not null anymore. But now getAction still returns null because the command is not loaded yet.

[FEA-eng]

It fixes the right-click crash, but it crashes or shows the errors below if I double-click on the subassembly.

19:18:49  Illegal storage access...
19:18:49  pyException: Traceback (most recent call last):
  File "<string>", line 2, in <module>
<class 'OSError'>: Illegal storage access! Please save your work under a new file name and restart the application!

OS: Windows 11 build 26200
Architecture: x86_64
Version: 1.2.0dev.46725 (Git)
Build date: 2026/05/06 16:16:18
Build type: Release
Branch: pr29887
Hash: 2dc85d867dd4cf5c24277155f202c5d44a15c417
Python 3.11.14, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.0, boost 1_84, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.0.0, OCC 7.8.1
Locale: Polish/Poland (pl_PL) [ OS: Polish/Poland (pl_PL) ]
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 96/81.5973/1

[PaddleStroke]

it crashes or shows the errors below if I double-click on the subassembly.

This is a different bug. See #29901

[FEA-eng]

@maxwxyz Tested, fixes the right-click crash