React doesn't recognize function

I am trying this tutorial for the react chat and keep getting the error

TypeError: n.props.handleNewUserMessage is not a function

I tried to resolve it using the following resources:

• https://www.reddit.com/r/reactjs/comments/6fd6nl/keep_getting_error_thispropsonsearch_in_not_a/?st=jhiugk4d&sh=fabd3cc4
• ReactJS with ES6: this.props is not a function when I communicate two components
• React TypeError this._test is not a function
• React does not recognize my function

This is my code:

import React, { Component } from 'react';
import { Widget, addResponseMessage } from 'react-chat-widget';
import 'react-chat-widget/lib/styles.css';

class App extends Component {
  componentDidMount() {
    addResponseMessage("How can I help you?");
  }

  handleNewUserMessage = (newMessage) => {
    console.log(`New message incomig! ${newMessage}`);
    // Now send the message throught the backend API
    addResponseMessage('response');
  }

  render() {
    return (
      <div className="App">
        <Widget />
      </div>
    );
  }
}


export default App;

Where have I gone wrong?