О странностях QtConcurrent+QFuture+QFutureWatcher

QString threadFunc()
{
  return QString("This is result");
}
 
QFutureWatcher<QString> watcher; // Глобальное объявление ???
QFuture<QString> future;
 
void MainWindow::on_pushButton_clicked()
{
  connect(&watcher,SIGNAL(finished()),
          this,   SLOT(on_finished()));
 
  watcher.setFuture(future);
  future = QtConcurrent::run(threadFunc);
}
 
void MainWindow::on_finished()
{
 QString text=  future.result();
 ui->textBrowser->insertPlainText(text);
}

void MainWindow::on_finished()
{
  QString text=  watcher.future().result(); //  через watcher !!!   Приложение падает
   ui->textBrowser->insertPlainText(text);
}

void MainWindow::on_pushButton_clicked()
{
  connect(&watcher,SIGNAL(finished()),
          this,   SLOT(on_finished()));
 
  watcher.setFuture(QtConcurrent::run(threadFunc));  // прямиком в watcher !!!
}

QNetworkReply* reply=  manager->get(request);
 
   connect( reply, SIGNAL(finished()),
            this, SLOT(replyFinished()) );

QFuture<quint64> future;
QFutureWatcher<quint64> watcher;
connect(&watcher, SIGNAL(finished()), this, SLOT(finished()));
watcher.setFuture(future);
qint32 n = 100;
future = QtConcurrent::run(fibonacci, n);