title: navigation.getChildNavigation is not a function
body: <p>Getting error after i update react-navigation package with > 2.2.5 (android/ios)</p>
  
  <pre><code>RN : 0.55.4
  react : 16.3.1
  react-navigation : 2.3.0
  redux: 3.7.2
  react-navigation-redux-helpers : 1.1.2
  </code></pre>
  
  <p>everything is working fine before updating react-navigation version </p>
  
  <p>similar issue on git hub but still not resolved 
  <a href="https://github.com/react-navigation/react-navigation/issues/4481" rel="noreferrer">https://github.com/react-navigation/react-navigation/issues/4481</a> </p>
  
  <p>my navigation structure </p>
  
  <pre><code>StackNavigation
                |
              home 
                |
              login
                |
                .
                .
  </code></pre>
  
  <p>I have only one navigation no nesting </p>
  
  <p><a href="https://i.stack.imgur.com/ZtUlt.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZtUlt.png" alt="enter image description here"></a></p>