MainWindow: Delete navigationwidget again

Change-Id: I4eecb9b93b4c490ea788da8c068a9a9fb9c3ac94
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -306,6 +306,9 @@ MainWindow::~MainWindow()
     m_helpManager = 0;
     delete m_variableManager;
     m_variableManager = 0;
+
+    delete m_navigationWidget;
+    m_navigationWidget = 0;
 }
 
 bool MainWindow::init(QString *errorMessage)