Draft: display: Fix crash when disabling a specific case of display mirroring
In case an unusable monitor (closed laptop lid) is present and the remaining monitors are mirrored, switching back from "Mirror" to "Join" crashes the app.
This issue was introduced in !1618 (merged) (4ed1af01) when the possibility to do this kind of mirroring was added.
Edited by Timotej Šulík