QProgressDialog causing crash when used inside of a QThread class

[agerlach]

All,

I have a class that inherits from QThread. In the run() method I have a for loop that performs my calculations. Because this takes several minutes I would like to use QProgressDialog. I am using it as follows

QProgressDialog progress(tr("Training the system..."),tr("Cancel"),0, numPts,0);
        progress.setCancelButton(0);
        progress.setWindowModality(Qt::WindowModal);
 
        for(int ii = 0; ii < numPts; ii++)
        {
            progress.setValue(ii);
            ...     
        }
        progress.setValue(numPts);
        progress.close();

QProgressDialog progress(tr("Training the system..."),tr("Cancel"),0, numPts,0);
        progress.setCancelButton(0);
        progress.setWindowModality(Qt::WindowModal);

        for(int ii = 0; ii < numPts; ii++)
        {
            progress.setValue(ii);
            ...     
        }
        progress.setValue(numPts);
        progress.close();

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When I run my code I get the following errors:

ASSERT failure in QWidget: "Widgets must be created in the GUI thread.", file kernel\qwidget.cpp, line 1231
ASSERT: "qApp && qApp->thread() == QThread::currentThread()" in file kernel\qapplication_win.cpp, line 929

If I comment out all the lines using progress the code runs fine.

The weird thing is that this is seemingly a function of numPts. When I use a dataset where I need to loop 54877 times (numPts) progress works. When I use a dataset where I need to loop 99116 times (numPts) progress causes the crash above.

I don't even know where to start on debugging this so any help is gratefully appreciated.