closeDialog: Make dialog inactive while fading out

Otherwise the user might click Kill, which would crash if the fade-out
was triggered because a Wayland window was closed.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1032

js/ui/dialog.js

@@ -58,10 +58,16 @@ class Dialog extends St.Widget {
         this._dialog.add_child(this.buttonLayout);
     }
 
-    _onDestroy() {
+    makeInactive() {
         if (this._eventId != 0)
             this._parentActor.disconnect(this._eventId);
         this._eventId = 0;
+
+        this.buttonLayout.get_children().forEach(c => c.set_reactive(false));
+    }
+
+    _onDestroy() {
+        this.makeInactive();
     }
 
     _modalEventHandler(actor, event) {