message: Allowed unmount of fluently navigated scene
  Fluently navigate from A to A --> B --> C then back twice to A. Scene B would throw an error because it had no navigationEvent in state and the crumbs from the global state navigator are less than B
diff: @@ -34,7 +34,7 @@ class Scene extends React.Component<SceneProps & {navigationEvent: NavigationEve
  var {crumb, navigationEvent: {stateNavigator}} = this.props;
  var {crumbs} = stateNavigator.stateContext;
  var stateContext = navigationEvent?.stateNavigator?.stateContext;
  - var {state, data} = stateContext || crumbs[crumb];
  + var {state, data} = stateContext || crumbs[crumb] || {};
  return (
  <NavigationContext.Provider value={navigationEvent}>
  {navigationEvent && this.props.renderScene(state, data)}
  