ReactJS - Uncaught TypeError：无法读取属性＆＃39; __ reactInternalInstance $ npc92gvg9bpy20iee3fpqfr＆＃39;＆＃39; null（...）

Question

我是比较新的反应，并从一些教程开始我被困在这一点，我无法确定为什么无法读取属性＆＃39; __ reactInternalInstance $ npc92gvg9bpy20iee3fpqfr＆＃39;抛出null 错误。这是我的代码：

var Profile = React.createClass({

getInitialState: function() {

    return {

        count: 0,

        task: '',

        gists: []


    };
},

addTask: function(e) {

    var items = this.state.items.concat([this.state.task]);

    var task = ''

    this.setState({ items, task });

    e.preventDefault();

},

onChange: function(e) {

    var task = e.target.value;

    this.setState({ task });
},

getDefaultProps: function() {

    return {
        path: 'http://lh3.googleusercontent.com/-CdI8n3eKqBY/AAAAAAAAAAI/AAAAAAAAAAA/AKTaeK-Iti8fjN610AAfj53Twp1R8PMNiQ/s96-c-mo/photo.jpg',

        items: []

    };
},

increment: function() {

    var count = this.state.count + 1;

    this.setState({ count });
},

decrement: function() {

    var count = this.state.count - 1;

    if(count < 0) {

        count = 0
    }

    this.setState({ count });
},

render: function() {

    var displayTask = (task) => <li>{ task }</li>;

    var newGist = function(gist) {

        return <Gist username={gist.username} url={gist.url} />
    };

    return (

        <div>

            <h1>Counter : { this.state.count }</h1>

                <button onClick = { this.increment }>+1</button>

                <button onClick = { this.decrement }>-1</button>

                <a href={ this.props.path }><img src={ this.props.path } /></a>

            <h1>My Tasks</h1>

                <Profile items={this.state.items} />

                <form onSubmit={this.addTask}>

                    <input type="text" placeholder="Enter New Task" onChange={this.onChange} value={this.state.task} />

                    <button>Add Task</button>

                </form>

                <ul>

                    { this.props.items.map(displayTask) }

                </ul>

            <h1>GistBox</h1>


            { this.state.gists.map(newGist) }

            {this.props.usernames}'s' last Gist is <a href={this.props.url}>here</a>.

        </div>

        );
    }
});

ReactDOM.render(<Profile />, document.body);