Fix more potential crashes in renderer widgets

Author: nyalldawson

src/gui/symbology-ng/qgsheatmaprendererwidget.cpp

@@ -133,7 +133,8 @@ QgsFeatureRendererV2* QgsHeatmapRendererWidget::renderer()
 void QgsHeatmapRendererWidget::setMapCanvas( QgsMapCanvas* canvas )
 {
   QgsRendererV2Widget::setMapCanvas( canvas );
-  mRadiusUnitWidget->setMapCanvas( canvas );
+  if ( mRadiusUnitWidget )
+    mRadiusUnitWidget->setMapCanvas( canvas );
 }
 
 void QgsHeatmapRendererWidget::applyColorRamp()

src/gui/symbology-ng/qgsinvertedpolygonrendererwidget.cpp

@@ -114,7 +114,8 @@ QgsFeatureRendererV2* QgsInvertedPolygonRendererWidget::renderer()
 void QgsInvertedPolygonRendererWidget::setMapCanvas( QgsMapCanvas* canvas )
 {
   QgsRendererV2Widget::setMapCanvas( canvas );
-  mEmbeddedRendererWidget->setMapCanvas( canvas );
+  if ( mEmbeddedRendererWidget )
+    mEmbeddedRendererWidget->setMapCanvas( canvas );
 }
 
 void QgsInvertedPolygonRendererWidget::on_mRendererComboBox_currentIndexChanged( int index )

src/gui/symbology-ng/qgspointdisplacementrendererwidget.cpp

@@ -171,8 +171,10 @@ QgsFeatureRendererV2* QgsPointDisplacementRendererWidget::renderer()
 void QgsPointDisplacementRendererWidget::setMapCanvas( QgsMapCanvas* canvas )
 {
   QgsRendererV2Widget::setMapCanvas( canvas );
-  mDistanceUnitWidget->setMapCanvas( canvas );
-  mEmbeddedRendererWidget->setMapCanvas( canvas );
+  if ( mDistanceUnitWidget )
+    mDistanceUnitWidget->setMapCanvas( canvas );
+  if ( mEmbeddedRendererWidget )
+    mEmbeddedRendererWidget->setMapCanvas( canvas );
 }
 
 void QgsPointDisplacementRendererWidget::on_mLabelFieldComboBox_currentIndexChanged( const QString& text )

src/gui/symbology-ng/qgssinglesymbolrendererv2widget.cpp

@@ -83,7 +83,8 @@ QgsFeatureRendererV2* QgsSingleSymbolRendererV2Widget::renderer()
 void QgsSingleSymbolRendererV2Widget::setMapCanvas( QgsMapCanvas* canvas )
 {
   QgsRendererV2Widget::setMapCanvas( canvas );
-  mSelector->setMapCanvas( canvas );
+  if ( mSelector )
+    mSelector->setMapCanvas( canvas );
 }
 
 void QgsSingleSymbolRendererV2Widget::changeSingleSymbol()