App.js introduces a subcomponent SnackbarExampleSimple

import React from "react";
import ReactDOM from "react-dom";
import SnackbarExampleSimple from "./Header"


  const App = () => (

    <div>
       {/* <Header/> */}
       <SnackbarExampleSimple/>
    </div>
   );
export default App;

the code in Header.js, is basically a copied official website instance

import React from "react";
// import ReactDOM from "react-dom";
// import "./css/header.css";

import Snackbar from "material-ui/Snackbar";
import RaisedButton from "material-ui/RaisedButton";

export default class SnackbarExampleSimple extends React.Component {

  constructor(props) {
    super(props);
    this.state = {
      open: false,
    };
  }

  handleClick = () => {
    this.setState({
      open: true,
    });
  };

  handleRequestClose = () => {
    this.setState({
      open: false,
    });
  };

  render() {
    return (
      <div>
        <RaisedButton
          onClick={this.handleClick}
          label="Add to my calendar"
        />
        <Snackbar
          open={this.state.open}
          message="Event added to your calendar"
          autoHideDuration={4000}
          onRequestClose={this.handleRequestClose}
        />
      </div>
    );
  }
}

A bunch of wrong reports