This error occurs on init of a Sencha Designer created app

Code:
 //NestedList:
        Ext.define('x.view.ui.Menu', {
            extend: 'Ext.dataview.NestedList',


            config: {
                store: 'MainMenuStore',
                toolbar: {
                    xtype: 'toolbar',
                    docked: 'top',
                    ui: 'light'
                }
            }


        });
        //MODEL:
        Ext.define('x.model.Menu', {
            extend: 'Ext.data.Model',
            config: {
                fields: [
                    {
                        name: 'text'
                    },
                    {
                        name: 'url'
                    }
                ]
            }
        });
        //STORE:
        Ext.define('x.store.base.MainMenuStore', {
            extend: 'Ext.data.TreeStore',
            requires: [
                'x.model.Menu'
            ],


            config: {
                model: 'x.model.Menu',
                storeId: 'MainMenuStore',
                proxy: {
                    type: 'ajax',
                    url: 'mainmenu.json',
                    reader: {
                        type: 'json',
                        rootProperty: 'items'
                    }
                }
            }
        });
        //JSON:
        {
            "text":"Main Menu",
            "items":[
                {
                    "text":"test",
                    "leaf":false,
                    "url":"test"
                }


            ]


        }
onProxyLoad: function(operation) {
var me = this,
records = operation.getRecords(),
successful = operation.wasSuccessful(),
node = operation.getNode();


node.beginEdit();Uncaught TypeError: Cannot call method 'beginEdit' of null