title: Event target is null inside functional setState
body: <p>Consider below function of some component:</p>
  <pre><code>handleInputChange(e) {
      // let val = e.target.value; - if I uncomment this, it works.
  
      // Update text box value
      this.setState(function (prevState, props) {
          return {
            searchValue: e.target.value,
          }
      })
  }
  </code></pre>
  <p>and a textbox, which is rendered by a child component of above component, and receives <code>handleInputChange</code> as <code>props</code>:</p>
  <pre><code>&lt;input type=&quot;text&quot; onChange={that.props.handleInputChange} value={that.props.searchValue} /&gt;
  </code></pre>
  <p>When I enter something in text field I get error that <code>Cannot read property 'value' of null</code>.</p>
  <p>If I uncomment the first line inside <code>handleInputChange</code> function, where I store text box value inside <code>val</code> variable, it works well. Ideas why?</p>