[bugfix] Crash on undo layout legend item on deleted item

[elpaso]

Fixes #19155

@wonder-sk please have a look: I think it's a safe move since layer() is not accessed in removeLegendFromLayer()

[wonder-sk]

I have looked into this and I think the correct fix should be in QgsLayerTreeLayer::layerWillBeDeleted() - by emitting layerWillBeUnloaded() signal before the weak pointer to the layer is cleared. Layer tree model listens to that signal and tries to clean up everything, but it can't because the layer pointer is null at the point when the slot is called.

We should also have a unit test for this case: 1. create a layer, 2. create a layer tree with the layer, 3. delete the layer, 4. check that layerLegendNodes() for the layer's node is empty.

[elpaso]

@wonder-sk bingo! Added a test (that was failing without the patch, as you suggested).

(I'll squash before commit if the code looks good to you)

[wonder-sk]

Looks good, thanks!