QmlProfiler: Initialize and reset QmlProfilerPlugin::m_profilerTool

Change-Id: I6efa81c1c89610f50f0057fccce438176bff3120
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/qmlprofiler/qmlprofilerplugin.cpp

@@ -112,6 +112,7 @@ void QmlProfilerPlugin::extensionsInitialized()
 ExtensionSystem::IPlugin::ShutdownFlag QmlProfilerPlugin::aboutToShutdown()
 {
     delete m_profilerTool;
+    m_profilerTool = nullptr;
 
     // Save settings.
     // Disconnect from signals that are not needed during shutdown