I updated Qt to the latest 6.6 repo. I have a styled widgets application and it started crashing in the QPlainTextEdit constructor. 

I've tracked the error to commit I5c32f5af8b284749732b610e56b4e3d8c8ed1946, QTBUG-1857 because of the change on line 2473 in qwidgettextcontrol.cpp. 

I've added a number of screenshots of the stacktrace.

In stacktrace2.png one sees that control is being constructed. Further in stacktrace5.png, q->setCursorWidth( minus 1 ) is called, which causes QApplication::style()>pixelMetric() to be called in stacktrace6.png. This is where the code was changed in the commit above, passing the parent widget. This causes QPlainTextEdit::isReadOnly() to be called in QStyleSheetStyle::renderRule line 2157, stacktrace8.png. At stacktrace9.png shows, d->control->textInteractionFlags() is tested. Howerver, d->control is still nullptr as it is still being constructed (stacktrace2.png).