Unloading Scene3D from Loader cause that app crashs (sometime).
I've investigated the problem in Qt Source and I found that the problem occurs in Scene3DRenderer (function "render()").
The Scene3DItem (m_item) is destroyed but the pointer is not set to nullptr. So when Scene3DRenderer try to "render", it crashs at line "const QSize boundingRectSize = m_item->boundingRect().size().toSize();" because m_item points to bad adress memory.
When Scene3DItem is destroyed, it will be good to stop render thread. I hope my above explanations whill help you to solve this problem.