title: Cannot read property 'setState' of null
body: <p>im starting ReactJS and i try to use Firebase as a database to collect my data. Im stuck from 2 days now cause of this error : "Cannot read property 'setState' of null"</p>
  
  <p>I can read my data from Firebase but i can't display them... I don't really know what to do :</p>
  
  <pre><code>import React from 'react';
  import ProductList from '../Product/ProductList';
  import Firebase from 'firebase';
  
  class HomePage extends React.Component {
    constructor() {
      super();
  
      this.state = {
        productList: []
      }
  
      var firebaseRef = new Firebase('https://codehunt-one.firebaseio.com/');
      firebaseRef.child("products").on('value', function(snapshot) {
        var products = snapshot.val();
        console.log(products);
  
        this.setState({
          productList: products
        })
      });
    }
  
    render() {
      return (
        &lt;section&gt;
          &lt;header&gt;
            &lt;img src="img/banner.jpeg" width="100%" /&gt;
          &lt;/header&gt;
  
          &lt;section&gt;
            &lt;section className="container"&gt;
                {this.state.productList
                  ?
                  &lt;ProductList productList={this.state.productList}/&gt;
                  :
                  null
                }
  
            &lt;/section&gt;
          &lt;/section&gt;
        &lt;/section&gt;
      );
    }
  }
  
  export default HomePage;
  </code></pre>