title: React Native Logout inside a DrawerNavigator
body: <p>I want to navigate back from DrawerNav to Login. Using alert('Alert') inside the function is OK.</p>
  
  <p>I have a StackNavigator with Login and DrawerNav</p>
  
  <pre><code>const MyStackNavigator = StackNavigator({
    Login : { screen : Login },
    DrawerNav : { screen : DrawerNav }
    }, {
      navigationOptions : { header : false }
    }
  );
  </code></pre>
  
  <p>From Login i can navigate to my main screen DrawerNav using</p>
  
  <pre><code>_login = () =&gt; {
    this.props.navigation.navigate('DrawerNav');
  }
  </code></pre>
  
  <p>Inside the DrawerNav is a DrawerNavigator (obviously)</p>
  
  <pre><code>const MyDrawerNavigator = DrawerNavigator({
      ... screens ...
    }, {
      initialRouteName : '',
      contentComponent : CustomContent,
      drawerOpenRoute : 'DrawerOpen',
      drawerCloseRoute : 'DrawerClose',
      drawerToggleRoute : 'DrawerToggle'
    }
  );
  
  const CustomContent = (props) =&gt; (
      &lt;View&gt;
        &lt;DrawerItems {...props} /&gt;
        &lt;TouchableOpacity
          onPress={ this._logout }&gt;
          &lt;Text style={ styles.logout }&gt;Logout&lt;/Text&gt;
        &lt;/TouchableOpacity&gt;
     &lt;/View&gt;
  )
  </code></pre>
  
  <p>As you can see, the logout is not part of the menu but inside the Drawer</p>
  
  <pre><code>_logout = () =&gt; {
    this.props.navigation.navigate('Login');
  }
  </code></pre>
  
  <p>This gives me an error</p>
  
  <pre><code>undefined is not an object (evaluating '_this.props.navigation')
  </code></pre>