fix(layertreeview): add checks on selectedModel to avoid segfault

[Djedouas]

Description

Fixes #59329

QGIS crashes when executing this line (and other from the same family) in the Python console:

QgsLayerTreeView().selectedLayers()

[github-actions]

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[rouault]

PyQgsLayerTreeView crashes on QT6 https://github.com/qgis/QGIS/actions/runs/11690407551/job/32556700307?pr=59337

[Djedouas]

@nyalldawson @rouault do you have an idea why the Qt6 test has a segfault? I can run the test in both Qt5 and Qt6 on my machine and it is fine...

Is seems as if the Qt6 test uses the code without my modifications...

[rouault]

I can run the test in both Qt5 and Qt6 on my machine and it is fine...

did you try running under Valgrind, just in case this might be something subtle... Or maybe it is due to a difference of Qt6 versions between your machine and the CI one ?

[nyalldawson]

@Djedouas looks like the test is actually hanging and timing out and being forcefully closed. But I can't see anything here that would lead to a hang... 🤔

I'd check if it's the new test causing this (as opposed to an existing test impacted by these changes) by temporarily removing it...

[github-actions]

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