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

Fixes #19155

src/core/layertree/qgslayertreemodel.cpp

@@ -884,14 +884,14 @@ void QgsLayerTreeModel::disconnectFromLayer( QgsLayerTreeLayer *nodeLayer )
 {
   disconnect( nodeLayer, nullptr, this, nullptr ); // disconnect from delayed load of layer
 
-  if ( !nodeLayer->layer() )
-    return; // we were never connected
-
   if ( testFlag( ShowLegend ) )
   {
     removeLegendFromLayer( nodeLayer );
   }
 
+  if ( !nodeLayer->layer() )
+    return; // we were never connected
+
   if ( _numLayerCount( mRootNode, nodeLayer->layerId() ) == 1 )
   {
     // last instance of the layer in the tree: disconnect from all signals from layer!