widget.get_effect(...) can return null while locking the screen, resulting
in a TypeError. In this situation the screen ends up black with a cursor
but never going to sleep, and moving the mouse brings the old screen
contents up but does not allow unlocking.
unlockDialog.js assumes that widget.get_effect will return non-null,
but other places such as getWindowDimmer in windowManager.js go out of
their way to be more careful.
[smcv: Add commit message, remove hard tabs, add missing semicolon]
Resolves: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3071
Bug-Debian: https://bugs.debian.org/968440
6b20eb8eea