[gnome-shell] authPrompt: Initialize property in _init()

[verdre <jonasd src gnome org>]

commit ff39b3274b72082e9a03165b7ec38f035f3210b2
Author: Florian Müllner <fmuellner gnome org>
Date:   Thu Feb 20 15:57:36 2020 +0100

    authPrompt: Initialize property in _init()
    
    This avoid a "reference to undefined property" warning when using
    it in a comparison.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1035

 js/gdm/authPrompt.js | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/js/gdm/authPrompt.js b/js/gdm/authPrompt.js
index 8cb1f305c9..0cb2462a0c 100644
--- a/js/gdm/authPrompt.js
+++ b/js/gdm/authPrompt.js
@@ -55,6 +55,7 @@ var AuthPrompt = GObject.registerClass({
 
         this._gdmClient = gdmClient;
         this._mode = mode;
+        this._defaultButtonWellActor = null;
 
         let reauthenticationOnly;
         if (this._mode == AuthPromptMode.UNLOCK_ONLY)