ERROR Error: Element type is invalid

hello im trying to try the example code and follow the step to install the package.

i try the write file example
`import {

AppRegistry,

} from 'react-native';
// require the module
var RNFS = require('react-native-fs');

// create a path you want to write to
// :warning: on iOS, you cannot write into `RNFS.MainBundlePath`,
// but `RNFS.DocumentDirectoryPath` exists on both platforms and is writable
const path = RNFS.DocumentDirectoryPath + '/test.txt';

// write the file
RNFS.writeFile(path, 'Lorem ipsum dolor sit amet', 'utf8')
.then((success) => {
console.log('FILE WRITTEN!');
})
.catch((err) => {
console.log(err.message);
});`

the app can built succesfully, but there is error

the metro console error is like this
ERROR Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: object. You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports.

This error is located at:
in RCTView (at View.js:34)
in View (at AppContainer.js:106)
in RCTView (at View.js:34)
in View (at AppContainer.js:132)
in AppContainer (at renderApplication.js:39)
[Sun Sep 06 2020 20:06:58.770] LOG FILE WRITTEN!

i check in my phone internal storage, but no file was written.
please help. thankyou