Add back in initialized toolbox XML

src/reducers/toolbox.js

 const UPDATE_TOOLBOX = 'scratch-gui/toolbox/UPDATE_TOOLBOX';
+import makeToolboxXML from '../lib/make-toolbox-xml';
 
 const initialState = {
-    toolboxXML: ''
+    // @todo switch this to make the stage's XML
+    toolboxXML: makeToolboxXML('')
 };
 
 const reducer = function (state, action) {