错误：App(...)：渲染没有返回任何内容

Question

<块引用>

What am I missing here?

错误：App(...)：渲染没有返回任何内容。这通常意味着缺少 return 语句。或者，不渲染任何内容，返回 null。 我是 react-native 的初学者，请帮我解决这个问题

import React, { useState } from 'react'; // 导入核心组件

import {
  AppRegistry,
  StyleSheet,
  Text,
  View,
  Button,
  Alert
} from 'react-native';

import DocumentPicker from 'react-native-document-picker';


const App = () => {
  
<Button title="select document" onPress={this.onDocumentPress} />


onDocumentPress = async () => {
  try {
    let urlOfS = 'https://yourbackendUrl.com'; // your url
    const res = await DocumentPicker.pick({
      type: [DocumentPicker.types.allFiles],
    })
    if (res.size < 50000000) {
      let data = new FormData();
      data.append('file', res)
      try {
        const responseOfFileUpload = await fetch(urlOfS, {
          method: 'POST',headers: {
            'Content-Type': 'multipart/form-data',
          },
          body: data,
        })
        if (responseOfFileUpload.status == 200) { 
          let responseInJs = await responseOfFileUpload.json();
          let fileName = responseInJs.fileName; // file name which will be        sent from backend
          showTopToast('Upload Succesfull');
        } else {
          showTopToast('Upload Failed');
        }
      } catch (err) {
        showTopToast('Upload Failed');
        console.log(err, 'error in upload');
      }
    } else {
      showTopToast('File size should not exceed 50 MB');
    }
    } catch (err) {if (DocumentPicker.isCancel(err)) {showTopToast('No document selected');
  } else {throw err;
  }
}}
};

export default App;