fix sandbox inspector

sandbox/public/index.html

@@ -23,6 +23,7 @@
 
     
     <script src="https://preview.babylonjs.com/inspector/babylon.inspector.bundle.js"></script>
+    <script src="https://preview.babylonjs.com/gui/babylon.gui.js"></script>
     <script src="dist/babylon.sandbox.js"></script>
     
     <style>