Fix a possible crash in FindReferences on shutdown

Cancel and wait for all threads started by FindReferences
in its destructor.

Task-number: QTCREATORBUG-25711
Change-Id: I83c3ea832a173a7fff5c52e0c0136dfff2234afe
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmljseditor/qmljsfindreferences.h

@@ -27,6 +27,8 @@
 
 #include "qmljseditor_global.h"
 
+#include <utils/futuresynchronizer.h>
+
 #include <QObject>
 #include <QFutureWatcher>
 #include <QPointer>
@@ -81,6 +83,7 @@ private:
 
     QPointer<Core::SearchResult> m_currentSearch;
     QFutureWatcher<Usage> m_watcher;
+    Utils::FutureSynchronizer m_synchronizer;
 };
 
 } // namespace QmlJSEditor