[bugfix] Crash on raster layer render change from python

Fixes #17517

Requires backporting

Author: elpaso

src/gui/raster/qgssinglebandgrayrendererwidget.cpp

@@ -180,17 +180,20 @@ void QgsSingleBandGrayRendererWidget::setFromRenderer( const QgsRasterRenderer *
   {
     //band
     mGrayBandComboBox->setBand( gr->grayBand() );
-    const QgsContrastEnhancement *ce = gr->contrastEnhancement();
-
     mGradientComboBox->setCurrentIndex( mGradientComboBox->findData( gr->gradient() ) );
-    //minmax
-    mDisableMinMaxWidgetRefresh = true;
-    mMinLineEdit->setText( QString::number( ce->minimumValue() ) );
-    mMaxLineEdit->setText( QString::number( ce->maximumValue() ) );
-    mDisableMinMaxWidgetRefresh = false;
-    //contrast enhancement algorithm
-    mContrastEnhancementComboBox->setCurrentIndex(
-      mContrastEnhancementComboBox->findData( ( int )( ce->contrastEnhancementAlgorithm() ) ) );
+
+    const QgsContrastEnhancement *ce = gr->contrastEnhancement();
+    if ( ce )
+    {
+      //minmax
+      mDisableMinMaxWidgetRefresh = true;
+      mMinLineEdit->setText( QString::number( ce->minimumValue() ) );
+      mMaxLineEdit->setText( QString::number( ce->maximumValue() ) );
+      mDisableMinMaxWidgetRefresh = false;
+      //contrast enhancement algorithm
+      mContrastEnhancementComboBox->setCurrentIndex(
+        mContrastEnhancementComboBox->findData( ( int )( ce->contrastEnhancementAlgorithm() ) ) );
+    }
 
     mMinMaxWidget->setFromMinMaxOrigin( gr->minMaxOrigin() );
   }