Fix crash in `simple_window` example #992

Job	Run time
cargo-deny	33s
format	22s
clippy-check	1m 23s
check-minimal	51s
Documentation on Github Pages	41s
coverage-tests	1m 36s
coverage-tests (server_system)	1m 40s
coverage-tests (client_system)	1m 39s
coverage-tests (client_system, server_system)	1m 18s
coverage-main	1m 46s
msrv	2m 54s
	14m 43s

Provide feedback