Avoid crash in style dock when layer is removed

src/app/qgsmapstylingwidget.cpp

@@ -17,6 +17,7 @@
 #include "qgsundowidget.h"
 #include "qgsrendererv2.h"
 #include "qgsrendererv2registry.h"
+#include "qgsmaplayerregistry.h"
 
 QgsMapStylingWidget::QgsMapStylingWidget( QgsMapCanvas* canvas, QWidget *parent )
     : QWidget( parent )
@@ -29,6 +30,8 @@ QgsMapStylingWidget::QgsMapStylingWidget( QgsMapCanvas* canvas, QWidget *parent
   layout->setContentsMargins( 0, 0, 0, 0 );
   this->setLayout( layout );
 
+  connect( QgsMapLayerRegistry::instance(), SIGNAL( layerWillBeRemoved( QgsMapLayer* ) ), this, SLOT( layerAboutToBeRemoved( QgsMapLayer* ) ) );
+
   mAutoApplyTimer = new QTimer( this );
   mAutoApplyTimer->setSingleShot( true );
   connect( mAutoApplyTimer, SIGNAL( timeout() ), this, SLOT( apply() ) );
@@ -199,6 +202,16 @@ void QgsMapStylingWidget::updateCurrentWidgetLayer( int currentPage )
   mBlockAutoApply = false;
 }
 
+void QgsMapStylingWidget::layerAboutToBeRemoved( QgsMapLayer* layer )
+{
+  if ( layer == mCurrentLayer )
+  {
+    mAutoApplyTimer->stop();
+    mStackedWidget->setCurrentIndex( mNotSupportedPage );
+    mCurrentLayer = nullptr;
+  }
+}
+
 
 QgsMapLayerStyleCommand::QgsMapLayerStyleCommand( QgsMapLayer *layer, const QDomNode &current, const QDomNode &last )
     : QUndoCommand()

src/app/qgsmapstylingwidget.h

@@ -49,6 +49,7 @@ class APP_EXPORT QgsMapStylingWidget : public QWidget
 
   private slots:
     void updateCurrentWidgetLayer( int currentPage );
+    void layerAboutToBeRemoved( QgsMapLayer* layer );
 
   private:
     int mNotSupportedPage;