Many render items/payloads contain smart pointers back to the
objects that added them to the scene, including entity and avatar
objects. Currently, those render items are destroyed when the
scene is destroyed very late in the application life-cycle.
There are rare crashes that can occur when these render items are
destroyed. Possibly, due to them referencing objects that have
already been destroyed via raw pointers. In an effort to
eliminate these crashes, we now destroy the scene earlier, within
Application::aboutToQuit() which is connected to the
QCoreApplication::aboutToQuit signal. Also, we guard against null
scene pointer dereferences. Any location that accesses the scene
off the main thread, now checks the validity of the scene pointer.
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