Qt GUI application crashes calling QThreadPool start two times

Question

I'm trying to use QThreadPool on a QRunnable-base class The code is the following

void Window::on_randomBtn_clicked()
{
    QString value;

    _pool.start(_randomizer);
    _pool.waitForDone();
    value = _randomizer->value();

    ui->RVal->setText(value);
    _red.setRgb(value.toInt(), 0, 0);


    _pool.start(_randomizer);
    _pool.waitForDone();
    value = _randomizer->value();

    ui->GVal->setText(value);
    _green.setRgb(0, value.toInt(), 0);
...

In the above code _pool is an instance of QThreadPool and _randomizer an instance of a class Randomizer which extends QRunnable. The run method of the Randomizer is just a oneliner producing random numbers

I can't understand why but what happens is just a SIGABRT signal received by the application after calling the second time the _pool.start(_randomizer)

I've also tried to instantiate two different Randomizer objects but the same behavior holds.

Could you help me explaining why?