导入react navigation和native base时出错

App.js

 
{import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
import {StackNavigator} from 'react-navigation'
import LoginScreen from './Screens/LoginScreen'
import HomeScreen from './Screens/HomeScreen'
export default class App extends React.Component {
  render() {
    return (
      <AppNavigator/>
    );
  }
}
const AppNavigator = StackNavigator({
  LoginScreen : {screen : LoginScreen},
  HomeScreen : {screen : HomeScreen}
})
const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
  },
});`

    LoginScreen.js

`
import React from 'react';
import { StyleSheet, Text, View } from 'react-native';
export default class LoginScreen extends React.Component {
  render() {
    return (
        <View>
     <Text> This is login Screen </Text>
     </View>
    );
  }
}
export default LoginScreen;`