title: React Navigation StackNavigator not appearing when used inside another view
body: <p>I'm attempting to use a React Navigation StackNavigator for a process that includes a static component at the top of the page and varying components at the bottom.  The code I'm using is:</p>
  
  <pre><code>const Navigator = StackNavigator ({
          splash: Splash,
          prompt: Prompt,
          pinCheck: PinCheck
  }, {
      initialRouteName: 'splash'
  });
  
  export default class Login extends React.Component
  {
      constructor (props)
      {
          super (props);
  
          // code to set up animation state    
      }
  
      componentDidMount()
      {
          // code to set up animation
      }
  
      finish_ (state)
      {
          this.props.navigation.navigate ('main', state);
      }
  
      render()
      {
          const screen = Dimensions.get('screen');
  
          return (
              &lt;KeyboardAvoidingView style={Global.styles.verticalFill} ref={this.saveContainerRef}&gt;
                  &lt;ScrollView style={{flex: 1}} contentContainerStyle={{justifyContent: 'space-between'}}&gt;
                      &lt;Animated.View style={{opacity:this.state.fade1,alignItems:'center'}} &gt;
                              &lt;Image
                                          style={{width:screen.width * 0.6,height: screen.height*0.55}}
                                          source={imgLogo}
                                          resizeMode='contain'
                                      /&gt;
                              &lt;Navigator /&gt;
                      &lt;/Animated.View&gt;
                  &lt;/ScrollView&gt;
              &lt;/KeyboardAvoidingView&gt;
          );
      }
  }
  </code></pre>
  
  <p>When I run this, however, my initial route component is not shown.  It works correctly if I swap <code>&lt;Navigator/&gt;</code> to <code>&lt;Splash/&gt;</code> however, so the component itself definitely works in this context.</p>
  
  <p>Any ideas what's wrong?</p>