Fix a crash in thumbnail generation

The dirty layers might have been removed from the image between
the dirty notification and the thumbnails update pass.

BUG:459510

plugins/dockers/layerdocker/LayerBox.cpp

@@ -1106,7 +1106,8 @@ void LayerBox::notifyThumbnailDirty()
 
 void LayerBox::updateDirtyThumbnails()
 {
-    Q_FOREACH (const QModelIndex &index, m_dirtyThumbnailNodes) {
+    Q_FOREACH (const QPersistentModelIndex &index, m_dirtyThumbnailNodes) {
+        if (!index.isValid()) continue;
         m_wdgLayerBox->listLayers->updateNode(index);
     }
     m_dirtyThumbnailNodes.clear();

plugins/dockers/layerdocker/LayerBox.h

@@ -187,7 +187,7 @@ private:
     KisNodeWSP m_savedNodeBeforeEditSelectionMode;
     bool m_blockOpacityUpdate {false};
     KisSignalAutoConnectionsStore m_activeNodeConnections;
-    QSet<QModelIndex> m_dirtyThumbnailNodes;
+    QSet<QPersistentModelIndex> m_dirtyThumbnailNodes;
     KisIdleWatcher m_idleWatcher;
 };