message: Checked for presence of scene in state
  Pressing back to home screen on android then going back in threw an
  error because state was empty. Not sure if this is what really happens
  or just in dev mode, but put the defensive checks in anyway
diff: @@ -50,10 +50,10 @@ class NavigationMotion extends React.Component<any, any> {
  }
  renderScene({key, data: {scene, state, data, mount}, style: {transitioning, ...transitionStyle}}) {
  var {mountedStyle, crumbStyle, children} = this.props;
  - var style = scene.style || getStyle(mount ? mountedStyle : crumbStyle, state, data);
  + var style = (scene && scene.style) || getStyle(mount ? mountedStyle : crumbStyle, state, data);
  return (
  <Motion key={key} style={transitioning ? transitionStyle : style}>
  - {(tweenStyle) => children(tweenStyle, scene.element, state, data)}
  + {(tweenStyle) => children(tweenStyle, scene && scene.element, state, data)}
  </Motion>
  );
  }
  @@ -67,11 +67,14 @@ class NavigationMotion extends React.Component<any, any> {
  <TransitionMotion
  willEnter={() => getStyle(unmountedStyle, state, data, 1, true)}
  willLeave={() => getStyle(unmountedStyle, oldState, oldData, 1)}
  - styles={sceneContexts.map(({state, data, url}) => ({
  + styles={sceneContexts.map(({state, data, url}) => {
  + var scene = this.state.scenes[url];
  + return {
  key: url,
  - data: {scene: this.state.scenes[url], state, data, mount: url === nextCrumb.url},
  - style: getStyle(this.state.scenes[url].style || mountedStyle, state, data, spring(0))
  - }))}>
  + data: {scene, state, data, mount: url === nextCrumb.url},
  + style: getStyle((scene && scene.style) || mountedStyle, state, data, spring(0))
  + }
  + })}>
  {tweenStyles => (
  <View style={style}>
  {tweenStyles.map((config) => this.renderScene(config))}
  