[gnome-shell] screenShield: Fix use of null this._dialog

[Florian Müllner <fmuellner src gnome org>]

commit 527ce66cd452aa64a0e64e5fc368698d64116192
Author: Matthew Leeds <matthew leeds endlessm com>
Date:   Thu Apr 23 17:42:16 2020 -0700

    screenShield: Fix use of null this._dialog
    
    On Fedora 32 if you close the laptop lid during gnome-initial-setup,
    gnome-shell hits this error:
    
    JS ERROR: Exception in callback for signal: prepare-for-sleep: TypeError: this._dialog is undefined
    _resetLockScreen@resource:///org/gnome/shell/ui/screenShield.js:434:9
    activate@resource:///org/gnome/shell/ui/screenShield.js:571:14
    lock@resource:///org/gnome/shell/ui/screenShield.js:617:14
    _prepareForSleep@resource:///org/gnome/shell/ui/screenShield.js:219:22
    _emit@resource:///org/gnome/gjs/modules/core/_signals.js:133:47
    _prepareForSleep@resource:///org/gnome/shell/misc/loginManager.js:198:14
    _emit@resource:///org/gnome/gjs/modules/core/_signals.js:133:47
    _convertToNativeSignal@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:169:19
    
    This is because _ensureUnlockDialog() hit its first early return. So
    return early from activate() in that case, so this._dialog doesn't get
    used while it's null.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1213

 js/ui/screenShield.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/js/ui/screenShield.js b/js/ui/screenShield.js
index 98279e070f..ab75577fa9 100644
--- a/js/ui/screenShield.js
+++ b/js/ui/screenShield.js
@@ -561,7 +561,8 @@ var ScreenShield = class {
         if (this._activationTime == 0)
             this._activationTime = GLib.get_monotonic_time();
 
-        this._ensureUnlockDialog(true);
+        if (!this._ensureUnlockDialog(true))
+            return;
 
         this.actor.show();