[bugfix] Crash when exporting (invalid) legend to qlr

Fixes #18981 Save QLR crashes QGIS 3

This patch definitely prevents the crash but I'm not sure if
we should also introduce the same check in QgsLayerTree::isLayer
or even in QgsLayerTreeGroup::findLayers() or also when importing
a qlr that might lead to invalid layer (like in this
bug report).

Author: elpaso

src/core/qgslayerdefinition.cpp

@@ -214,6 +214,11 @@ bool QgsLayerDefinition::exportLayerDefinition( QDomDocument doc, const QList<Qg
   QList<QgsLayerTreeLayer *> layers = root->findLayers();
   Q_FOREACH ( QgsLayerTreeLayer *layer, layers )
   {
+    if ( ! layer->layer() )
+    {
+      QgsDebugMsgLevel( QStringLiteral( "Not a valid map layer: skipping %1" ).arg( layer->name( ) ), 4 );
+      continue;
+    }
     QDomElement layerelm = doc.createElement( QStringLiteral( "maplayer" ) );
     layer->layer()->writeLayerXml( layerelm, doc, context );
     layerselm.appendChild( layerelm );