@Tee_
normally a pro file results in a separate target (executable or library). Even if you "combine" them with a subdirs project.
So your AppUi project must link against the AppLib

For someone facing a similar issue, I did not find a direct solution,

According to the table
https://doc.qt.io/qt-5/qtqml-cppintegration-data.html#sequence-type-to-javascript-array
QList<CustomClass> is not yet supported in QML. Due to this reason I had to convert the QList<QList<CustomClass>> to QVariantMap<string,QVariantList<QVariantList<QVariantMap>>>

And while using the values in qml use the bracket notation instead of the dot notation.

I just noticed that XYSeries is a uncreatable type, its derived object's Q_INVOKABLE method invoke member method of DeclarativeXySeries class, and DeclarativeXySeries is a shadow class about XYSeries. Such as LineSeries is imported from DeclarativeLineSeries.

@TTIO Did you ever fix this? I am having the exact same problem and am thoroughly stumped.

@Ankit.Jain
such errors appear when you try to access c.listOfA too early before it has a proper value assigned

Sorry, everything is ok now. It's not about the typo in the forum, but in my project.
Tried to build minimal working example -- working without any problems.