Error when adding popup to project

[AllanCodes]

Hello, I get this error when I add to my index.js file.

Target container is not a DOM element.

ReactDOM.render(<Main />, document.getElementById('root'));
ReactDOM.render(<Popup />, document.getElementById('popupContainer'));

Any ideas what the issue can be? Should I create the element popupContainer somewhere?

[tbleckert]

Do you have an element with the id 'popupContainer' in your HTML?

[AllanCodes]

I've now added popupContainer and get no error, however placing this above my class definition

Popup.alert('Hello'); doesn't do anything. I need mention that my ReactDOM.render is in my index.js file and the Popup.alert() is in another class file

[tbleckert]

You need to style the component, to start with you can use the demo css