message: Checked for navigateEvent in handleBack
  When reloading on non-zero crumb in android then navigateEvent is null (scene blank) and still want to press back to return to original crumb and start over
diff: @@ -7,6 +7,7 @@ class NavigationMotion extends React.Component {
  constructor(props) {
  super(props);
  this.state = {navigationEvent: null};
  + this.handleBack = this.handleBack.bind(this);
  }
  static defaultProps = {
  crumb: 0,
  @@ -51,20 +52,20 @@ class NavigationMotion extends React.Component {
  return stateContext;
  }
  componentDidMount() {
  - if (this.props.crumb) {
  - this.handleBack = () => {
  - this.state.navigationEvent.stateNavigator.navigateBack(1);
  - return true;
  - }
  + if (this.props.crumb)
  BackHandler.addEventListener('hardwareBackPress', this.handleBack);
  }
  - }
  shouldComponentUpdate(props, state) {
  return state.navigationEvent === props.navigationEvent;
  }
  componentWillUnmount() {
  if (this.props.crumb)
  BackHandler.removeEventListener('hardwareBackPress', this.handleBack);
  + }
  + handleBack() {
  + if (this.state.navigationEvent)
  + this.state.navigationEvent.stateNavigator.navigateBack(1);
  + return !!this.state.navigationEvent;
  }
  render() {
  var {navigationEvent} = this.state;
  