mainwindow: stop preview before closing the main window

Change-Id: Ibbe60290952b9a546541622f709bbc663aa45a47

mainwindow.cpp

@@ -316,6 +316,7 @@ void MainWindow::closeEvent(QCloseEvent* event)
         this->hide();
         event->ignore();
     } else {
+        LRCInstance::avModel().stopPreview();
         settings.setValue(SettingsKey::geometry, saveGeometry());
         settings.setValue(SettingsKey::windowState, saveState());
         this->disconnect(screenChangedConnection_);