In the following code QStylePrivate::proxyStyle is dereferenced uncheckedly if it isn't equal to this but ** it can be nullptr under some circumstances:

const QStyle * QStyle::proxy() const
{
    Q_D(const QStyle);
    return d->proxyStyle == this ? this : d->proxyStyle->proxy();
} 

At least I had a problem with QWindowsVistaStyle.