Fix exit crash when having context help open.

Add a shutdown()-notification to MainWindow and
reset the context in it.

Task-number: 256386
Reviewed-by: con <qtc-committer@nokia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -1081,6 +1081,13 @@ void MainWindow::resetContext()
     updateContextObject(0);
 }
 
+void MainWindow::shutdown()
+{
+    disconnect(QApplication::instance(), SIGNAL(focusChanged(QWidget*,QWidget*)),
+               this, SLOT(updateFocusWidget(QWidget*,QWidget*)));
+    m_activeContext = 0;
+}
+
 static const char *settingsGroup = "MainWindow";
 static const char *geometryKey = "Geometry";
 static const char *colorKey = "Color";