QmlProfiler: Initialize and reset QmlProfilerPlugin::m_profilerTool

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

src/plugins/qmlprofiler/qmlprofilerplugin.h

@@ -51,7 +51,7 @@ public:
     QList<QObject *> createTestObjects() const override;
 
 private:
-    QmlProfilerTool *m_profilerTool;
+    QmlProfilerTool *m_profilerTool = nullptr;
 };
 
 } // namespace Internal