QMetaObject::newInstance returns a QObject, thus it's not possible to
create a new instance of a Q_GADGET using this function. Previously, we
returned a non-null QObject pointer for such scenarios, which then
leads to crashes when one tries to use it. Now, we check whether the
meta object inherits QObject's meta object, and error out early
otherwise.
Change-Id: I7b1fb6c8d48b3e98161894be2f281a491963345e
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
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