unlockDialog: Only cancel AuthPrompt if it exists

AuthPrompt is created on demand, and this._authPrompt is
expected to be null except on very strictly controlled
occasions. The idle monitor callback isn't one of them.

Check if AuthPrompt exists before cancelling it.

Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2371

js/ui/unlockDialog.js

@@ -761,7 +761,7 @@ var UnlockDialog = GObject.registerClass({
     }
 
     _escape() {
-        if (this.allowCancel)
+        if (this._authPrompt && this.allowCancel)
             this._authPrompt.cancel();
     }