Crash in QQmlBinding::getPropertyData with null propertyCache

We have the following code in plasma:

• we have an engine  where a qobject is exposed via an attached property

• we create a new engine with that exposes that same QObject as a context property

• we delete the second engine, causing the (shared) QQmlInfo:propertyCache to be set to nullptr [1]

• when the first engine updates a binding on that object, we crash accessing the null propertyCache.

[1] Introduced here https://codereview.qt-project.org/#/c/192037/3/src/qml/jsruntime/qv4qobjectwrapper.cpp

This happens with latest 5.9 branch.

A simplified demo is attached.