message: Pevented error when empty state context
  For example, don't want to navigate to invisible tab on ios. Wait until it's pressed
diff: @@ -64,7 +64,7 @@ class NavigationStack extends React.Component<NavigationStackProps> {
  style={styles.stack}
  {...this.getAnimation()}
  onDidNavigateBack={this.onDidNavigateBack}>
  - {crumbs.concat(nextCrumb).map((_, crumb) => (
  + {nextCrumb && crumbs.concat(nextCrumb).map((_, crumb) => (
  <Scene
  key={crumb}
  crumb={crumb}
  