Updating a QProgressBar while doing long paint

We have a lengthy paint for the main widget of our application, like couple tens of seconds at max.

We have a progress bar in the bottom status bar that show how the painting process is going. This worked fine under Qt3. Now with Qt4, we have a crash inside QTransform::type (we can plainly see that the 'this' is zero at that point).

The overall sequence of events is this:

• a QPainter gets created
• we start painting with it
• we call QProgressBar::setValue() (works fine)
• next painting operation on the original QPainter results in a crash

What can we do to have a working progress bar while painting in Qt4 ?