React native native modules with swift getting exported method value as undefined?

I am creating native module using create-react-native-module with swift. After that I have followed react native official documentation for iOS setup. doc link:: https://facebook.github.io/react-native/docs/native-modules-ios

I have created create-react-native-module with example. I am just adding simple function with returning string "Hello World" inside my native module.

My 'CustomModule-Bridging-Header.h'::

#import <React/RCTBridgeModule.h>

My 'CustomModule.m'::

#import <React/RCTBridgeModule.h>

@interface RCT_EXTERN_MODULE(CustomModule, NSObject)

RCT_EXTERN_METHOD(sampleMethod)

+ (BOOL) requiresMainQueueSetup {
  return YES;
}

@end

My 'CustomModule.swift':

import Foundation

@objc(CustomModule)
class CustomModule: NSObject {
  @objc(sampleMethod)
  func sampleMethod() -> String {
      return "Hello World"
  }
}

After making these changes to native module I have installed dependencies again inside example. Now my App.js is look like::

import React, { Component } from 'react';
import { Platform, Text, View } from 'react-native';
import CustomModule from 'react-native-custom-module';

export default class App extends Component {
    state = {
        message: '--'
    };

    async componentDidMount() {
        const msg = await CustomModule.sampleMethod();
        console.log('javascript calling msg::', msg);
        this.setState({
            message: msg
        });
    }
    render() {
        return (
            <View>
                <Text>CustomModule example☆</Text>
                <Text>{this.state.message}</Text>
            </View>
        );
    }
}

Here I am getting "msg" value as undefined. Please help!