* add import to clarify source of helper
It took me a while to figure out where 'addNavigationHelpers' comes from, maybe adding the import in the doc here will help the next guy.
* add spaces around imported value
* removed empty line
When the drawer opens / close, the result of AppNavigator.router.getStateForAction(action, state); is null. So the nav state in the store is null which causes errors. This solves that.
