If you want to compare a default constructed QJsonDocument to one that contains a QJsonObject, it will crash.

    QJsonDocument doc1;
    QJsonDocument doc2;
    const bool iseq1 = doc1 == doc2; // no crash here

    QJsonObject obj;
    doc1.setObject(obj);
    const bool iseq2 = doc1 == doc2; // here it crashes

If we are comparing them the other way, it is working fine.
I suspect that in operator==, only this is checked for null, but the right-side argument is not.