所以我是React Native的新手，但是每次我从Facebook React Native Doc中放置一个新的组件，比如Button或Image，我都会得到一个“ReferenceError：模拟器上出现“找不到变量”错误？当我添加除Text之外的任何其他代码时，都会发生这种情况。我是不是搞错了什么？什么都行。

/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 * @flow
 */

import React, { Component } from 'react';

import {
  Platform,
  StyleSheet,
  Text,
  View
} from 'react-native';

const instructions = Platform.select({
  ios: 'Press Cmd+R to reload,\n' +
    'Cmd+D or shake for dev menu',
  android: 'Double tap R on your keyboard to reload,\n' +
    'Shake or press menu button for dev menu',
});

export default class App extends Component<{}> {
  render() {
    return (
        <View style={styles.container}>
        
        <Text style={styles.welcome}>
          This is a new App!
        </Text>
        <Text style={styles.instructions}>
          To get started, edit App.js
        </Text>
        <Text style={styles.instructions}>
          {instructions}
        </Text>
        <Button
  onPress={onPressLearnMore}
  title="Learn More"
  color="#841584"
  accessibilityLabel="Learn more about this purple button"
/>
        <Text style={{color: 'blue'}}>Hello</Text>
      </View>
        
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: 'white',
  },
  welcome: {
    fontSize: 20,
    textAlign: 'center',
    margin: 10,
  },
  instructions: {
    textAlign: 'center',
    color: '#333333',
    marginBottom: 5,
  },
});

**错误This is the error