title: open pupop when select a tab react native
body: <p>i'm new to react native and i want to have a tabNavigator that when 'Other' tab is selected a popup shows up . its my code but it don't recognize 'this' in 'ModalExample' class.
  any help would be appreciated.</p>
  
  <pre><code>export const Tabs = TabNavigator({
      Notifications: {
          screen: Notifications,
          navigationOptions: {
              tabBarIcon: ({ tintColor }) =&gt; &lt;Icon name="access-alarms" size={25} color={tintColor} /&gt;
          },
      },
      Home: {
          screen: Home,
          navigationOptions: {
              tabBarIcon: ({ tintColor }) =&gt; &lt;Icon name="home" size={25} color={tintColor} /&gt;
          },
      },
  
      Other: {
          screen: ModalExample,
          navigationOptions: {
              tabBarIcon: ({ tintColor }) =&gt; &lt;Icon name="more" size={25} color={tintColor} /&gt;
          },
      },
  },
  {
      tabBarOptions: {
          showIcon: true,
          showLabel: false,
          scrollEnabled: false
      },
      tabBarPosition: 'bottom',
  });
  </code></pre>
  
  <p>and its other class:</p>
  
  <pre><code>export default class ModalExample extends Component {
  constructor(props){
      super(props);
  
      this.popupDialog.show();
  }
  
  render() {
      return (
          &lt;View&gt;
              &lt;PopupDialog
                  ref={(popupDialog) =&gt; { this.popupDialog = popupDialog; }}
              &gt;
                  &lt;View&gt;
                      &lt;Text&gt;Hello&lt;/Text&gt;
                  &lt;/View&gt;
              &lt;/PopupDialog&gt;
          &lt;/View&gt;
      );
  }
  </code></pre>
  
  <p>}</p>