layers: add null check

Xaver Huglrequested to merge
work/cherry-pick-0a4e7d89 into Plasma/6.2

This fixes https://crash-reports.kde.org/organizations/kde/issues/96985

(cherry picked from commit 0a4e7d89)

Co-authored-by: Xaver Hugl xaver.hugl@gmail.com

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