Deferred raycasting sometimes throws an error or panics when the window is closed

[TimLeach635]

It's possible to get into a situation where the raycaster throws an error when the window is closed. This doesn't affect gameplay, of course, but I'm seeing it in the logs and it's not ideal.

To reproduce, this happens every time I run the mouse_picking_deferred example, and close the window:

This is probably quite minor, but it would be good if this didn't happen!