layout: Clean up consoleBackgroundGroup

We shouldn't leave a reference to a destroyed actor just hanging
around.

https://bugzilla.gnome.org/show_bug.cgi?id=694227

js/ui/layout.js

@@ -607,6 +607,8 @@ const LayoutManager = new Lang.Class({
         global.stage_input_mode = Shell.StageInputMode.NORMAL;
 
         this._consoleBackgroundGroup.destroy();
+        this._consoleBackgroundGroup = null;
+
         this._startingUp = false;
 
         this.trayBox.show();