Having a Qt 5.15.5 that was built for x86, Windows, QWebEngineView and QGraphicsScene (used for off-screen rendering)
Rendering a QWebEngineView by render() method, and everything works-as-designed.
But, in some undefined scenarios, it throws some nullpointer exception within QOpenGLFunctions::glReadPixels: a pixels is nullptr.
I think that's may be a some strange bug that should be fixed, passing some screenshots of exception points
Point first: calling QGraphicsScene render() method

And then the point where exception happens

There's a full traceback:

And a proof that pixels pointer is nullptr

An important note: the problem occurs not all the time, but maybe once in 35-40 minutes, for sure.