title: undefined is not an object (evaluating 'allRowsIDs.length') (React-Native)
body: <p>I'm very new to React-Native. Following the basic React-Native tutorial I'm trying to fetch JSON data from "<a href="https://facebook.github.io/react-native/movies.json" rel="noreferrer">https://facebook.github.io/react-native/movies.json</a>". I can display the <code>title</code> and <code>description</code> properties but when I try to display the "movies" property using a <code>ListView</code> I get the following error:</p>
  
  <blockquote>
    <p>undefined is not an object (evaluating 'allRowsIDs.length')</p>
  </blockquote>
  
  <pre><code>import React, { Component } from 'react';
  import {
    AppRegistry,
    StyleSheet,
    Text,
    View,
    ListView
  } from 'react-native';
  
  var ds = new ListView.DataSource({rowHasChanged: (r1, r2) =&gt; r1 !== r2});
  
  export default class AwesomeProject extends Component {
    constructor(props) {
      super(props);
      this.state = { 
          dataSource: 'init',
      };
    }
  
    componentWillMount() {
      fetch('https://facebook.github.io/react-native/movies.json')
        .then((response) =&gt; response.json())
        .then((responseJson) =&gt; {
          this.setState({ dataSource: ds.cloneWithRows(responseJson.movies) });
        })
        .catch((error) =&gt; {
          console.error(error);
        });
    }
  
    render() {
        return (
          &lt;View style={styles.container}&gt;
            &lt;ListView
              dataSource={this.state.dataSource}
              renderRow={(rowData) =&gt; &lt;Text&gt;{rowData}&lt;/Text&gt;}
            /&gt;
          &lt;/View&gt;
        );
    }
  }
  </code></pre>