Setting the QBrush.style to Qt:Dense4Pattern, using the brush, and painting to a QPrinter device (save to PDF or use printer driver), causes apps to crash.
QPrinter* printer = new QPrinter(QPrinter::ScreenResolution);
if( !printer )

{ return; }

printer->setDocName("test");
printer->setOutputFileName(QString());
printer->setFromTo(0, 0);

QPrintDialog dlg(printer, nullptr);
if( dlg.exec() != QDialog::Accepted ){ return; }

QPainter p(printer);
if( !p.isActive() )

{ return; }

QBrush checkerBrush;
checkerBrush.setColor(Qt::white);
//QT BUG: The next line causes the program to crash
// during printing (PDF or printer device).
// Behavior observed on MacOS 10.14.1 Mojave
// under Qt 5.9.6
checkerBrush.setStyle(Qt::Dense4Pattern);
p.setBrush(checkerBrush);
p.setRenderHint(QPainter::Antialiasing, true);

QRect rect(10, 20, 80, 60);
p.drawRect(rect);

I've provided source code for the Basic Drawing app, tweaked to include the above code in RenderArea::setBrush.