the non-const QJsonArray::operator[] requires a valid index and all the
const operator[] say that it returns Undefined if the key or index
doesn't exist. This is the exception.
Task-number: QTBUG-39864
Change-Id: Iaa63461109844e978376fffd15f9716f0cc66cca
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
c798b286bd