title: instance member is not accessible
body: <p>How can I access class functions from inside stack navigator header? Is this possible?
  What I'm trying to achieve is to call a function when I press the stack navigator header title.</p>
  
  <pre><code>class Dashboard extends React.Component {
          static navigationOptions = ({ navigation }) =&gt; {
              return {
                  headerTitle: (
                          &lt;View style={header.addressView}&gt;
                              &lt;Text
                                  style={header.titleAddress}
                                  onPress={() =&gt; {
                                    this._show().bind(this);
                                  }}&gt;
                              /&gt;
                          &lt;/View&gt;
                  ),
              };
          };
  
          _show(){
              this.setState({ visibleModal: 1 })
          }
  
          constructor(props) {
              super(props);
  
              this.state = {
                  visibleModal: null,
              };
          }
  
          render() {
              ...........
  
          }
      }
  
      export default Dashboard;
  </code></pre>