teacher guide could be completely removed from application

src/adaptors/UBThumbnailAdaptor.cpp

@@ -165,7 +165,7 @@ void UBThumbnailAdaptor::persistScene(UBDocumentProxy* proxy, UBGraphicsScene* p
 
         pScene->render(&painter, imageRect, sceneRect, Qt::KeepAspectRatio);
 
-        if(UBApplication::boardController->paletteManager()->teacherGuideDockWidget()->teacherGuideWidget()->isModified()){
+        if(UBApplication::boardController->paletteManager()->teacherGuideDockWidget() && UBApplication::boardController->paletteManager()->teacherGuideDockWidget()->teacherGuideWidget()->isModified()){
             QPixmap toque(":images/toque.svg");
             painter.setOpacity(0.6);
             painter.drawPixmap(QPoint(width - toque.width(),0),toque);