save project without project file while quitting

Signed-off-by: Hugo Beauzée-Luyssen <hugo@beauzee.fr>

src/Gui/MainWindow.cpp

@@ -791,7 +791,7 @@ MainWindow::closeEvent( QCloseEvent* e )
         switch ( ret )
         {
         case QMessageBox::Save:
-            Core::getInstance()->project()->save();
+            on_actionSave_triggered();
             break;
         case QMessageBox::Discard:
             break;