D20486: Avoid crash with on alt+tab(tabbox) view on close it current window

[ZC]

zccrs added a reviewer: zzag.
zccrs changed the visibility from "Only User: zccrs (ZC)" to "All Users".
zccrs changed the edit policy from "All Users" to "Administrators".
Herald added a subscriber: kwin.

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D20486

To: zccrs, #kwin, zzag
Cc: kwin, LeGast00n, The-Feren-OS-Dev, sbergeron, jraleigh, fbampaloukas, GB_2, mkulinski, ragreen, jackyalcine, iodelay, crozbo, bwowk, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, romangg, jensreuterberg, abetts, sebas, apol, ahiemstra, mart
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