`Output` in Smithay doesn't track if the output still exists, other than
based on whether or not it has strong references. Which doesn't seem to
be working correctly.
There may be leaked strong references to `Output`s somewhere, and maybe
Smithay should track if an output is still valid, generally when it is
exposed as a Wayland global
(https://github.com/Smithay/smithay/issues/1584). But a check like this
works for now.
Addresses https://github.com/pop-os/cosmic-comp/issues/985.
