title: How can i hide and show stack navigator header in react-native?
body: <p>How can I hide and then show header(stack navigator) by pressing a button ?</p>
  
  <pre><code>static navigationOptions = ({ navigation }) =&gt; {
      return {
          header : null
      }
  }
  </code></pre>
  
  <p>This code set header to <code>null</code> and hide header but i can't show it again.</p>