Make sure to add NodeInterface fields in your model

MyTreeStoreExample.js

/**
Custom model to a TreeStore. It's important to define text and qtip!
*/
Ext.define('MyApp.model.MyTreeModel',{
  extend: 'Ext.data.Model',
  fields: [{
    name: 'text',
    type: 'string'
  },{
    name: 'leaf',
    type: 'boolean'
  },{
    name: 'qtip',
    type: 'string'
  },{
    name: 'myCustomField'
  }]
});

Ext.define('MyApp.store.MyTree',{
  extend: 'Ext.data.TreeStore',
  model: 'MyApp.model.MyTreeModel', //here if your model don't have text and leaf, it will not work properly
  autoLoad: true,
  root : {
    text: 'Root',
    expanded: true
  },
  proxy : {
    type: 'ajax',
    url: 'myurl.json',
    reader : {
      type: 'json'
    }
  }
});