Avoid crash when disconnecting layer

src/app/qgsgeometryvalidationdock.cpp

@@ -265,6 +265,7 @@ void QgsGeometryValidationDock::onCurrentLayerChanged( QgsMapLayer *layer )
   {
     disconnect( mCurrentLayer, &QgsVectorLayer::editingStarted, this, &QgsGeometryValidationDock::onLayerEditingStatusChanged );
     disconnect( mCurrentLayer, &QgsVectorLayer::editingStopped, this, &QgsGeometryValidationDock::onLayerEditingStatusChanged );
+    disconnect( mCurrentLayer, &QgsVectorLayer::destroyed, this, &QgsGeometryValidationDock::onLayerDestroyed );
   }
 
   mCurrentLayer = qobject_cast<QgsVectorLayer *>( layer );
@@ -273,6 +274,7 @@ void QgsGeometryValidationDock::onCurrentLayerChanged( QgsMapLayer *layer )
   {
     connect( mCurrentLayer, &QgsVectorLayer::editingStarted, this, &QgsGeometryValidationDock::onLayerEditingStatusChanged );
     connect( mCurrentLayer, &QgsVectorLayer::editingStopped, this, &QgsGeometryValidationDock::onLayerEditingStatusChanged );
+    connect( mCurrentLayer, &QgsVectorLayer::destroyed, this, &QgsGeometryValidationDock::onLayerDestroyed );
   }
 
   onLayerEditingStatusChanged();
@@ -297,6 +299,12 @@ void QgsGeometryValidationDock::onLayerEditingStatusChanged()
   mTopologyChecksPendingButton->setEnabled( enabled );
 }
 
+void QgsGeometryValidationDock::onLayerDestroyed( QObject *layer )
+{
+  if ( layer == mCurrentLayer )
+    mCurrentLayer = nullptr;
+}
+
 void QgsGeometryValidationDock::showHighlight( const QModelIndex &current )
 {
   QgsVectorLayer *vlayer = qobject_cast<QgsVectorLayer *>( mMapCanvas->currentLayer() );

src/app/qgsgeometryvalidationdock.h

@@ -48,6 +48,7 @@ class QgsGeometryValidationDock : public QgsDockWidget, public Ui_QgsGeometryVal
     void onCurrentErrorChanged( const QModelIndex &current, const QModelIndex &previous );
     void onCurrentLayerChanged( QgsMapLayer *layer );
     void onLayerEditingStatusChanged();
+    void onLayerDestroyed( QObject *layer );
     void gotoNextError();
     void gotoPreviousError();
     void zoomToProblem();