Bug#968440: gnome-shell 3.36.5-1 crashes on screen lock.

[Zephaniah E. Loss-Cutler-Hull]

Package: gnome-shell
Version: 3.36.5-1

When the screen locks on gnome-shell 3.36.5-1, I get the following 
errors in my logs, and 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 let me unlock.

Doing a 'killall -HUP gnome-shell' by ssh brings it back, but that's 
obviously not a real solution.

JS ERROR: TypeError: widget.get_effect(...) is null
_updateBackgroundEffects at resource:///org/gnome/shell/ui/unlockDialog.js:651:20
_updateBackgrounds at resource:///org/gnome/shell/ui/unlockDialog.js:667:14
_init at resource:///org/gnome/shell/ui/unlockDialog.js:531:14
_ensureUnlockDialog at resource:///org/gnome/shell/ui/screenShield.js:379:28
activate at resource:///org/gnome/shell/ui/screenShield.js:564:14
lock at resource:///org/gnome/shell/ui/screenShield.js:620:14
_onStatusChanged/this._lockTimeoutId<@resource:///org/gnome/shell/ui/screenShield.js:263:26
JS ERROR: TypeError: widget.get_effect(...) is null
_updateBackgroundEffects at resource:///org/gnome/shell/ui/unlockDialog.js:651:20
_updateBackgrounds at resource:///org/gnome/shell/ui/unlockDialog.js:667:14
_init at resource:///org/gnome/shell/ui/unlockDialog.js:531:14
_ensureUnlockDialog at resource:///org/gnome/shell/ui/screenShield.js:379:28
activate at resource:///org/gnome/shell/ui/screenShield.js:564:14
_onLongLightbox at resource:///org/gnome/shell/ui/screenShield.js:308:18
onComplete at resource:///org/gnome/shell/ui/lightbox.js:195:18
_makeEaseCallback/<@resource:///org/gnome/shell/ui/environment.js:75:13
_easeActor/<@resource:///org/gnome/shell/ui/environment.js:149:56

Taking a look at the source, unlockDialog.js at line 651 as indeed 
assuming that widget.get_effect will return non-null, and other places 
such as getWindowDimmer in windowManager.js around line 156 go out of 
their way to be more careful.

The attached patch fixes the issue on my system.

Regards,
Zephaniah E. Loss-Cutler-Hull.
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