QQuickRendercontrol crashes when Animators are being used

In our project, we have the following architecture:

a list of (potentially multiple) QQuickRenderControl instances, together with the assoaciated machinery (offscreen QQuickWindow, QQmlEngine etc.)

These can be created and disposed of dynamically at runtime (depending on what patch(es) the user loads).

In general, this works exceptionally well, but we encountered a crash when the QML source in question contains animations.

I attached a repro project, which is a simplified excerpt from our implementation and which shows the observed behaviour:

1. hit "start" to create a new instance of the rendering object
-> you should see the bouncing ball
2. hit stop to dispose of the just created rendering object
-> you should crash

it does not happen all the time; you might have to repeat the above steps a couple of times for the crash to happen.

If you comment out the animation stuff in the referenced QML file, the crash will not happen.

I made the follwoing attempt to get rid of the crash (but wouldn't succeed):
before actuially destructing the root item, I was traversing the object tree and I would send stop() to every animation element I found - unfortunately, this did not change anything.