title: Error when using nested navigators in react-navigation & FluidTransitions
body: <p>I'm confused on how to declare my nested react-navigation navigator.</p>
  
  <p>So the structure I'm trying to make it is a set of screens in the BottomTabNavigator, and this BottomTabNavigator is in a FluidNavigator itself. Now, the HomeScreen is under BottomTabNavigator, but a child component in BottomTabNavigator is using <a href="https://github.com/fram-x/FluidTransitions" rel="nofollow noreferrer">Fluid Transitions</a>' Shared Element Transition. Due to this structure, I'm receiving the error:</p>
  
  <blockquote>
    <p>TypeError: expected dynamic type 'double', but had type 'null'</p>
  </blockquote>
  
  <p>When I remove the tabs system, everything's working fine. So I guess something's wrong with the way I'm creating these Navigators.</p>
  
  <p>Here's the structure I'm using:</p>
  
  <pre><code>&gt; Fluid Navigator:
    &gt; Home (Bottom Tab Navigator)
      &gt; SharedScreenSource
      &gt; Tab2
      &gt; Tab3
    &gt; SharedScreenDestination
  </code></pre>
  
  <p>Here's the current code for reference:</p>
  
  <pre><code>const HomeTabsNav = createBottomTabNavigator(
  {
      Tab1WithSharedScreenComponent: { screen: Tab1, path: '/tab1' },
      Tab2: { screen: Tab2, path: '/tab2' },
      Tab3: { screen: Tab3, path: '/tab3' }
  },
  { tabBarPosition: 'bottom', animationEnabled: false, swipeEnabled: false }
  );
  
  const RootStack = createFluidNavigator(
  {
      Home: HomeTabsNav,
      SharedScreenDestination: SharedScreenDestination // this screen opens through Tab1's child component
  },
  {
      initialRouteName: 'Home',
      mode: 'card',
      headerMode: 'none',
      navigationOptions: {
          header: null
      },
  }
  );
  </code></pre>