Element type not valid: string expected (for inline components) or React Error class / function

I have an issue with React when trying to change a page using a button (but) the console gives me:

Element type is invalid: expected string (for built-in components) or> class / function (for composite components), but received: undefined. You> probably forgot to export your component from the file it defined.

But when I paste the code onClick

into the class code, it shows the jsx code. This is how it works, but why am I still getting the error? Here is my code:

import React from 'react';
import {render} from 'react-dom';

//sass
require('../sass/style.scss');

var pages = ['Qrscan'];
var jsx = [<Qrscan />];

class App extends React.Component {
    constructor (props) {
        super(props);
        this.state = {target: <Menu />};
        App.changePage = App.changePage.bind(this);
    }

    static changePage (tar) {
       this.setState({target: tar});
    }

    render () {
        return (
          this.state.target
        )
    }
}


//Rendering
class Menu extends React.Component {
    render() {
        return (
            <div id="root">
                <Header />
                <div className="container">
                    <But target="Qrscan" class="button button-QR" text="Use QR code" type="button" linkTo="" />
                    <But class="button button-URL" text="Use URL" type="button" />
                </div>
            </div>
        )
    }
}

class Qrscan extends React.Component {
    render() {
        return (
            <h1>test</h1>
        )
    }
}

//elements
class Header extends React.Component {
    render () {
        return (
            <div className="header">
                <div className="header-logo">Jukey</div>
            </div>
        )
    }
}

class But extends React.Component {
    getInfo () {
        for (var i = 0; i < pages.length; i++) {
            if(pages[i] == this.props.target) {
                return jsx[i];
            }
        }
    }

    render () {
        return (
            <button onClick={() => App.changePage(this.getInfo())} className={this.props.class}>{this.props.text}</button>
        )
    }
}

render (<App />, document.getElementById('app'));

      

        
        
        
      

    

Thanks for taking the time to help me with my problems!