[gnome-shell/wip/smcv/issue3071-3.36] unlockDialog: Defend against absence of blur effect

[Simon McVittie <smcv src gnome org>]

commit 9363624b9902916037fb4fad063832362f9a86b4
Author: Zephaniah E. Loss-Cutler-Hull <warp aehallh com>
Date:   Sat Aug 15 03:23:27 2020 -0700

    unlockDialog: Defend against absence of blur effect
    
    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

 js/ui/unlockDialog.js | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)
---
diff --git a/js/ui/unlockDialog.js b/js/ui/unlockDialog.js
index a074c6fc4c..9ae8266225 100644
--- a/js/ui/unlockDialog.js
+++ b/js/ui/unlockDialog.js
@@ -648,10 +648,14 @@ var UnlockDialog = GObject.registerClass({
         const themeContext = St.ThemeContext.get_for_stage(global.stage);
 
         for (const widget of this._backgroundGroup.get_children()) {
-            widget.get_effect('blur').set({
-                brightness: BLUR_BRIGHTNESS,
-                sigma: BLUR_SIGMA * themeContext.scale_factor,
-            });
+            let effect = widget.get_effect('blur');
+
+            if (effect) {
+                effect.set({
+                    brightness: BLUR_BRIGHTNESS,
+                    sigma: BLUR_SIGMA * themeContext.scale_factor,
+                });
+            }
         }
     }