Fix crash when editing graduated symbol in style dock

Fixes #18909

(cherry-picked from 9887508)

Author: nyalldawson

python/gui/auto_generated/symbology/qgssymbolselectordialog.sip.in

@@ -28,6 +28,7 @@ Symbol selector widget that can be used to select and build a symbol
 %End
   public:
 
+
     QgsSymbolSelectorWidget( QgsSymbol *symbol, QgsStyle *style, QgsVectorLayer *vl, QWidget *parent /TransferThis/ = 0 );
 %Docstring
 Symbol selector widget that can be used to select and build a symbol

src/gui/symbology/qgscategorizedsymbolrendererwidget.cpp

@@ -548,6 +548,8 @@ void QgsCategorizedSymbolRendererWidget::changeCategorizedSymbol()
   std::unique_ptr<QgsSymbol> newSymbol( mCategorizedSymbol->clone() );
   if ( panel && panel->dockMode() )
   {
+    // bit tricky here - the widget doesn't take ownership of the symbol. So we need it to last for the duration of the
+    // panel's existence. Accordingly, just kinda give it ownership here, and clean up in cleanUpSymbolSelector
     QgsSymbolSelectorWidget *dlg = new QgsSymbolSelectorWidget( newSymbol.release(), mStyle, mLayer, panel );
     dlg->setContext( mContext );
     connect( dlg, &QgsPanelWidget::widgetChanged, this, &QgsCategorizedSymbolRendererWidget::updateSymbolsFromWidget );

src/gui/symbology/qgsgraduatedsymbolrendererwidget.cpp

@@ -977,7 +977,9 @@ void QgsGraduatedSymbolRendererWidget::changeRangeSymbol( int rangeIdx )
   QgsPanelWidget *panel = QgsPanelWidget::findParentPanel( this );
   if ( panel && panel->dockMode() )
   {
-    QgsSymbolSelectorWidget *dlg = new QgsSymbolSelectorWidget( newSymbol.get(), mStyle, mLayer, panel );
+    // bit tricky here - the widget doesn't take ownership of the symbol. So we need it to last for the duration of the
+    // panel's existence. Accordingly, just kinda give it ownership here, and clean up in cleanUpSymbolSelector
+    QgsSymbolSelectorWidget *dlg = new QgsSymbolSelectorWidget( newSymbol.release(), mStyle, mLayer, panel );
     dlg->setContext( mContext );
     connect( dlg, &QgsPanelWidget::widgetChanged, this, &QgsGraduatedSymbolRendererWidget::updateSymbolsFromWidget );
     connect( dlg, &QgsPanelWidget::panelAccepted, this, &QgsGraduatedSymbolRendererWidget::cleanUpSymbolSelector );

src/gui/symbology/qgssymbolselectordialog.h

@@ -93,6 +93,8 @@ class GUI_EXPORT QgsSymbolSelectorWidget: public QgsPanelWidget, private Ui::Qgs
 
   public:
 
+    // TODO QGIS 4.0 - transfer ownership of symbol to widget!
+
     /**
        * Symbol selector widget that can be used to select and build a symbol
        * \param symbol The symbol to load into the widget as a start point.