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 !!!
}

future = QtConcurrent::run(threadFunc);
watcher.setFuture(future);

// Instantiate the objects and connect to the finished signal.
MyClass myObject;
QFutureWatcher<int> watcher;
connect(&watcher, SIGNAL(finished()), &myObject, SLOT(handleFinished()));
 
// Start the computation.
QFuture<int> future = QtConcurrent::run(...);
watcher.setFuture(future);

connect(&watcher, SIGNAL(finished()), &myObject, SLOT(handleFinished()));

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

int pos = 0;
 
void MainWindow::on_finished()
{
    QString text= watcher->result(pos++);
    ui->textBrowser->insertPlainText(text);
}

typedef QFutureWatcher<QString> Watcher;
//----------------------------------------
QString threadFunc(int n)
{
  QString msg=
   QString("Thread №%1 \t task №%2  finished.\r\n")
     .arg(QString::number(
             (int)QThread::currentThreadId()))// Thread HANDLE, not Id
     .arg(QString::number(n));
  return msg;
}
//----------------------------------------
void MainWindow::createTask(int n)
{
  Watcher* watcher= new Watcher;
  watcher->setFuture(QtConcurrent::run(threadFunc,n));
 
  QThread::sleep(30);  /* Имитируем что тут "вклинился" другой поток.
                            и забрал процессорное время*/
  /*через 30 cек после запуска */
  connect(watcher,SIGNAL(finished()),
          this,   SLOT(on_finished()));
}
//----------------------------------------
void MainWindow::on_pushButton_clicked()
{
    createTask(0);
}
//---------------------------------------
void MainWindow::on_finished()
{
 Watcher* watcher=
    static_cast<Watcher*>(sender());
 Q_ASSERT(watcher);
 
 QString text= watcher->result();
 ui->textBrowser->insertPlainText(text);
  watcher->deleteLater();
}
//---------------------------------------