windowNotDefined.js

class TextEditor extends Component{
  constructor(props) {
    if (typeof window !== 'undefined') {
      this.CKEditor = require('@ckeditor/ckeditor5-react')
      this.ClassicEditor = require('@ckeditor/ckeditor5-build-classic')
    }
  }
  render(){
    const CKEditor = this.CKEditor
    const ClassicEditor = this.ClassicEditor
  
    return(
      <CKEditor
        editor={ClassicEditor}
        ....
       />
     )
  }
}