Followup c9251c, fix crash

Author: nyalldawson

python/core/layertree/qgslayertreemodellegendnode.sip

@@ -156,7 +156,7 @@ class QgsSymbolLegendNode : QgsLayerTreeModelLegendNode
      * render context in advance and call this method instead of minimumIconSize().
      * @note added in QGIS 2.18
      */
-    QSize minimumIconSize( QgsRenderContext &context ) const;
+    QSize minimumIconSize( QgsRenderContext* context ) const;
 
     /** Returns the symbol used by the legend node.
      * @see setSymbol()

src/core/layertree/qgslayertreemodel.cpp

@@ -1497,7 +1497,7 @@ void QgsLayerTreeModel::invalidateLegendMapBasedData()
       QgsSymbolLegendNode* n = dynamic_cast<QgsSymbolLegendNode*>( legendNode );
       if ( n )
       {
-        const QSize sz( n->minimumIconSize( *context ) );
+        const QSize sz( n->minimumIconSize( context.data() ) );
         const QString parentKey( n->data( QgsLayerTreeModelLegendNode::ParentRuleKeyRole ).toString() );
         widthMax[parentKey] = qMax( sz.width(), widthMax.contains( parentKey ) ? widthMax[parentKey] : 0 );
         n->setIconSize( sz );

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -159,25 +159,25 @@ Qt::ItemFlags QgsSymbolLegendNode::flags() const
 QSize QgsSymbolLegendNode::minimumIconSize() const
 {
   QScopedPointer<QgsRenderContext> context( createTemporaryRenderContext() );
-  return minimumIconSize( *context );
+  return minimumIconSize( context.data() );
 }
 
-QSize QgsSymbolLegendNode::minimumIconSize( QgsRenderContext& context ) const
+QSize QgsSymbolLegendNode::minimumIconSize( QgsRenderContext* context ) const
 {
   QSize minSz( 16, 16 );
   if ( mItem.symbol() && mItem.symbol()->type() == QgsSymbol::Marker )
   {
     minSz = QgsImageOperation::nonTransparentImageRect(
               QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( 512, 512 ),
-                  &context ).toImage(),
+                  context ).toImage(),
               minSz,
               true ).size();
   }
   else if ( mItem.symbol() && mItem.symbol()->type() == QgsSymbol::Line )
   {
     minSz = QgsImageOperation::nonTransparentImageRect(
               QgsSymbolLayerUtils::symbolPreviewPixmap( mItem.symbol(), QSize( minSz.width(), 512 ),
-                  &context ).toImage(),
+                  context ).toImage(),
               minSz,
               true ).size();
   }