QtQuick: Unknown method return type QAbstractItemModel *

I have a QObject-derived class which returns a QAbstractItemModel * from a Q_INVOKABLE function.  When trying to assign the result of this function to a parameter in a QML object, I get:

Error: Unknown method return type: QAbstractItemModel*

What really got me was that I didn't have this error for a long time, with all the same C++ code. Then I happened to remove an "import QtQuick.Controls 1.4" from another, seemingly unrelated, component which was also being loaded at the same time (into the same context). Only then did I start getting this error.

If I change my C++ code to return a QObject* instead of a QAbstractItemModel* then the QML works again, w/out complaining.  But of course I'd rather return the actual type (also for C++ usage, etc).

The affected code is pretty basic, but I haven't tried isolating a standalone example.

In C++:

Q_INVOKABLE QAbstractItemModel * ModuleFactory::widgetsModel(const QByteArray &parentId = QByteArray(), QObject *parent = nullptr);

In QML (ModuleFactory is a singleton):

import QtQuick 2.10
Menu {
  ...
  readonly property var dataModel: ModuleFactory.widgetsModel();
}

I also tried adding QtQml and QtQml.Models imports, but no difference.  If I add QtQuickControls 1.4 import, it works.

Please let me know if I can provide more information or if I'm missing something.

Well a good clue is that the only reference to registering QAbstractItemModel which I could find is in Controls 1... https://code.woboq.org/qt5/qtquickcontrols/src/controls/plugin.cpp.html#178

So maybe this is by design, but it seems strange.  Obviously I can register the type myself as a workaround, but the class "belongs" to Qt after all.

Thank you,
-Max