The EXC_BAD_ACCESS is fixed, it was actually the problem hinted at here: Fail to use vtk on mac 10.15
QSurfaceFormat::setDefaultFormat(QVTKOpenGLNativeWidget::defaultFormat()); mentioned in QVTKOpenGLNativeWidget.h fixed it.
Now, I’ve got still another problem though - the QVTKOpenGLNativeWidgets don’t yet show what they are supposed to show; they show a copy of some other parts of the screen…